build: Convert warnings into errors when testing for -fstack-clash-protection

When building with Clang, if `-fstack-clash-protection` is used with an
unsupported target, it may result in hundreds of
`-Wunused-command-line-argument` warnings at compile time. This is
currently the case when building for at least Darwin using Apple or LLVM
Clang.

Unsupported targets may also include *BSD, however that is changing; see
further discussion in https://reviews.llvm.org/D92245 and
https://reviews.freebsd.org/D27366. 

Note that this option is already skipped for Windows.

Author: hebasto

configure.ac

@@ -894,7 +894,7 @@ if test x$use_hardening != xno; then
       dnl See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458 for more details.
       ;;
     *)
-      AX_CHECK_COMPILE_FLAG([-fstack-clash-protection],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-clash-protection"])
+      AX_CHECK_COMPILE_FLAG([-fstack-clash-protection], [HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-clash-protection"], [], [$CXXFLAG_WERROR])
       ;;
   esac