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.

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../../ \