Refactor build system to use libpcre2 as the default

Updated the build system and related source files to use libpcre2 as the
default regex library instead of the deprecated libpcre. This change
ensures future compatibility and aligns with the library's maintenance status.

To build with the old libpcre, the `--with-pcre` configuration parameter
can be specified.

configure.ac

@@ -141,15 +141,16 @@ CHECK_LIBXML2
 
 
 #
-# Check for libpcre
+# Check for libpcre only if explicitly requested
 #
-CHECK_PCRE
-
-
-#
-# Check for pcre2
-#
-PROG_PCRE2
+if test "x${with_pcre}" != "x" && test "x${with_pcre}" != "xno"; then
+  CHECK_PCRE
+else
+  #
+  # Check for pcre2
+  #
+  PROG_PCRE2
+fi
 
 
 # Checks for header files.