Simplifiy configuration to build libModSecurity with std C++17

- Leveraged autoconf again to check whether the C++ compiler supports
  the required standard version and build using it.
- Replaced the outdaded `ax_cxx_compile_stdcxx_11.m4` macro with the
  latest version of `ax_cxx_compile_stdcxx` which supports C++17.
  - https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
  - https://raw.githubusercontent.com/autoconf-archive/autoconf-archive/e4e5269db2764b9f53d759c24750ac6ca38e02ea/m4/ax_cxx_compile_stdcxx.m4
- This should also streamline updating to C++20 in the future.

build/ax_cxx_compile_stdcxx_11.m4

@@ -1,171 +0,0 @@
-# ============================================================================
-#  http://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html
-# ============================================================================
-#
-# SYNOPSIS
-#
-#   AX_CXX_COMPILE_STDCXX_11([ext|noext],[mandatory|optional])
-#
-# DESCRIPTION
-#
-#   Check for baseline language coverage in the compiler for the C++11
-#   standard; if necessary, add switches to CXXFLAGS to enable support.
-#
-#   The first argument, if specified, indicates whether you insist on an
-#   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
-#   -std=c++11).  If neither is specified, you get whatever works, with
-#   preference for an extended mode.
-#
-#   The second argument, if specified 'mandatory' or if left unspecified,
-#   indicates that baseline C++11 support is required and that the macro
-#   should error out if no mode with that support is found.  If specified
-#   'optional', then configuration proceeds regardless, after defining
-#   HAVE_CXX11 if and only if a supporting mode is found.
-#
-# LICENSE
-#
-#   Copyright (c) 2008 Benjamin Kosnik <bkoz@redhat.com>
-#   Copyright (c) 2012 Zack Weinberg <zackw@panix.com>
-#   Copyright (c) 2013 Roy Stogner <roystgnr@ices.utexas.edu>
-#   Copyright (c) 2014, 2015 Google Inc.; contributed by Alexey Sokolov <sokolov@google.com>
-#   Copyright (c) 2015 Paul Norman <penorman@mac.com>
-#
-#   Copying and distribution of this file, with or without modification, are
-#   permitted in any medium without royalty provided the copyright notice
-#   and this notice are preserved. This file is offered as-is, without any
-#   warranty.
-
-#serial 12
-
-m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [[
-  template <typename T>
-    struct check
-    {
-      static_assert(sizeof(int) <= sizeof(T), "not big enough");
-    };
-
-    struct Base {
-    virtual void f() {}
-    };
-    struct Child : public Base {
-    virtual void f() override {}
-    };
-
-    typedef check<check<bool>> right_angle_brackets;
-
-    int a;
-    decltype(a) b;
-
-    typedef check<int> check_type;
-    check_type c;
-    check_type&& cr = static_cast<check_type&&>(c);
-
-    auto d = a;
-    auto l = [](){};
-    // Prevent Clang error: unused variable 'l' [-Werror,-Wunused-variable]
-    struct use_l { use_l() { l(); } };
-
-    // http://stackoverflow.com/questions/13728184/template-aliases-and-sfinae
-    // Clang 3.1 fails with headers of libstd++ 4.8.3 when using std::function because of this
-    namespace test_template_alias_sfinae {
-        struct foo {};
-
-        template<typename T>
-        using member = typename T::member_type;
-
-        template<typename T>
-        void func(...) {}
-
-        template<typename T>
-        void func(member<T>*) {}
-
-        void test();
-
-        void test() {
-            func<foo>(0);
-        }
-    }
-
-    // Check for C++11 attribute support
-    void noret [[noreturn]] () { throw 0; }
-]])
-
-AC_DEFUN([AX_CXX_COMPILE_STDCXX_11], [dnl
-  m4_if([$1], [], [],
-        [$1], [ext], [],
-        [$1], [noext], [],
-        [m4_fatal([invalid argument `$1' to AX_CXX_COMPILE_STDCXX_11])])dnl
-  m4_if([$2], [], [ax_cxx_compile_cxx11_required=true],
-        [$2], [mandatory], [ax_cxx_compile_cxx11_required=true],
-        [$2], [optional], [ax_cxx_compile_cxx11_required=false],
-        [m4_fatal([invalid second argument `$2' to AX_CXX_COMPILE_STDCXX_11])])
-  AC_LANG_PUSH([C++])dnl
-  ac_success=no
-  AC_CACHE_CHECK(whether $CXX supports C++11 features by default,
-  ax_cv_cxx_compile_cxx11,
-  [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
-    [ax_cv_cxx_compile_cxx11=yes],
-    [ax_cv_cxx_compile_cxx11=no])])
-  if test x$ax_cv_cxx_compile_cxx11 = xyes; then
-    ac_success=yes
-  fi
-
-  m4_if([$1], [noext], [], [dnl
-  if test x$ac_success = xno; then
-    for switch in -std=gnu++11 -std=gnu++0x; do
-      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx11_$switch])
-      AC_CACHE_CHECK(whether $CXX supports C++11 features with $switch,
-                     $cachevar,
-        [ac_save_CXXFLAGS="$CXXFLAGS"
-         CXXFLAGS="$CXXFLAGS $switch"
-         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
-          [eval $cachevar=yes],
-          [eval $cachevar=no])
-         CXXFLAGS="$ac_save_CXXFLAGS"])
-      if eval test x\$$cachevar = xyes; then
-        CXXFLAGS="$CXXFLAGS $switch"
-        ac_success=yes
-        break
-      fi
-    done
-  fi])
-
-  m4_if([$1], [ext], [], [dnl
-  if test x$ac_success = xno; then
-    dnl HP's aCC needs +std=c++11 according to:
-    dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
-    for switch in -std=c++11 -std=c++0x +std=c++11; do
-      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx11_$switch])
-      AC_CACHE_CHECK(whether $CXX supports C++11 features with $switch,
-                     $cachevar,
-        [ac_save_CXXFLAGS="$CXXFLAGS"
-         CXXFLAGS="$CXXFLAGS $switch"
-         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
-          [eval $cachevar=yes],
-          [eval $cachevar=no])
-         CXXFLAGS="$ac_save_CXXFLAGS"])
-      if eval test x\$$cachevar = xyes; then
-        CXXFLAGS="$CXXFLAGS $switch"
-        ac_success=yes
-        break
-      fi
-    done
-  fi])
-  AC_LANG_POP([C++])
-  if test x$ax_cxx_compile_cxx11_required = xtrue; then
-    if test x$ac_success = xno; then
-      AC_MSG_ERROR([*** A compiler with support for C++11 language features is required.])
-    fi
-  else
-    if test x$ac_success = xno; then
-      HAVE_CXX11=0
-      AC_MSG_NOTICE([No compiler with C++11 support was found])
-    else
-      HAVE_CXX11=1
-      AC_DEFINE(HAVE_CXX11,1,
-                [define if the compiler supports basic C++11 syntax])
-    fi
-
-    AC_SUBST(HAVE_CXX11)
-  fi
-])

configure.ac

@@ -52,8 +52,8 @@ AC_PROG_MAKE_SET
 PKG_PROG_PKG_CONFIG
 
 
-# Check if the compiler is c++17 compatible.
+# Set C++ standard version and check if compiler supports it.
-# AX_CXX_COMPILE_STDCXX_17(,mandatory)
+AX_CXX_COMPILE_STDCXX(17, noext, mandatory)
 
 # Check for libinjection
 if ! test -f "${srcdir}/others/libinjection/src/libinjection_html5.c"; then

examples/reading_logs_via_rule_message/Makefile.am

@@ -32,7 +32,6 @@ simple_request_LDFLAGS = \
 
 simple_request_CPPFLAGS = \
 	$(GLOBAL_CFLAGS) \
-	-std=c++17 \
 	-I$(top_builddir)/headers \
 	-I$(top_builddir) \
 	-g \

examples/reading_logs_with_offset/Makefile.am

@@ -32,7 +32,6 @@ read_LDFLAGS = \
 
 read_CPPFLAGS = \
 	$(GLOBAL_CFLAGS) \
-	-std=c++17 \
 	-I$(top_builddir)/headers \
 	-I$(top_builddir) \
 	-g \

examples/using_bodies_in_chunks/Makefile.am

@@ -33,7 +33,6 @@ simple_request_LDFLAGS = \
 
 simple_request_CPPFLAGS = \
 	$(GLOBAL_CFLAGS) \
-	-std=c++17 \
 	-I$(top_builddir)/headers \
 	-I$(top_builddir) \
 	-g \

src/Makefile.am

@@ -311,7 +311,6 @@ libmodsecurity_la_CFLAGS =
 
 
 libmodsecurity_la_CPPFLAGS = \
-	-std=c++17 \
 	-I.. \
 	-g \
 	-I../others \

src/parser/Makefile.am

@@ -11,7 +11,6 @@ libmodsec_parser_la_SOURCES = \
 	test.cc
 
 libmodsec_parser_la_CPPFLAGS = \
-	-std=c++17 \
 	-I../.. \
 	-I../../headers \
 	-I../../others \

test/Makefile.am

@@ -70,7 +70,6 @@ unit_tests_LDFLAGS = \
 
 
 unit_tests_CPPFLAGS = \
-	-std=c++17 \
 	-Icommon \
 	-I../ \
 	-g \
@@ -126,7 +125,6 @@ regression_tests_LDFLAGS = \
 
 
 regression_tests_CPPFLAGS = \
-	-std=c++17 \
 	-Icommon \
 	-I../ \
 	-g \
@@ -179,7 +177,6 @@ rules_optimization_LDFLAGS = \
 	$(YAJL_LDFLAGS)
 
 rules_optimization_CPPFLAGS = \
-	-std=c++17 \
 	-Icommon \
 	-I../ \
 	-g \

test/benchmark/Makefile.am

@@ -32,7 +32,6 @@ benchmark_LDFLAGS = \
 	$(LUA_LDFLAGS)
 
 benchmark_CPPFLAGS = \
-	-std=c++17 \
 	-I$(top_builddir)/headers \
 	$(GLOBAL_CPPFLAGS) \
 	$(PCRE_CFLAGS) \

test/fuzzer/Makefile.am

@@ -30,7 +30,6 @@ afl_fuzzer_LDADD = \
 
 
 afl_fuzzer_CPPFLAGS = \
-	-std=c++17 \
 	-Icommon \
 	-I../ \
 	-I../../ \

tools/rules-check/Makefile.am

@@ -28,7 +28,6 @@ modsec_rules_check_LDFLAGS = \
 	$(YAJL_LDFLAGS)
 
 modsec_rules_check_CPPFLAGS = \
-	-std=c++17 \
 	-I$(top_builddir)/headers \
 	$(GLOBAL_CPPFLAGS) \
 	$(PCRE_CFLAGS) \