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.

configure.ac

@@ -52,8 +52,8 @@ AC_PROG_MAKE_SET
 PKG_PROG_PKG_CONFIG
 
 
-# Check if the compiler is c++17 compatible.
-# AX_CXX_COMPILE_STDCXX_17(,mandatory)
+# Set C++ standard version and check if compiler supports it.
+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) \