Enable diagnose_if compile-time wrong dmax warnings

sizeof(dest)!=dmax checks as user-defined warnings (clang).
when __builtin_object_size returns the known object size.
See GH #40

e.g.:
test_strcoll_s.c:81:39: warning: wrong dmax [-Wuser-defined-warnings]
    rc = strcoll_s(str1, 0, str2, &ind);

configure.ac

@@ -374,6 +374,7 @@ ax_cv_check_ldflags___Wl__fatal_warnings=no
 dnl sprintf_s passing through to vsnprintf_s
 ax_cv_check_cflags__Wformat_2=no
 ax_cv_check_cflags__Wformat_nonliteral=no
+ax_cv_check_cflags__Wgcc_compat=no
 dnl windows mult. decl are not properly sorted out yet
 dnl const -Wdiscarded-qualifiers is stricter on mingw
 case $host_os in
@@ -443,6 +444,11 @@ case $host_os in
     # AM_LDFLAGS=`echo $AM_LDFLAGS|sed -e"s/-Wl,--as-needed -Wl,--no-as-needed//"`
     ;;
 esac
+dnl against -Werror,-Wgcc-compat
+case $WARN_CFLAGS in
+  *-Wformat-security*)
+      WARN_CFLAGS="$WARN_CFLAGS -Wno-gcc-compat -Wno-error=user-defined-warnings" ;;
+esac
 
 dnl Was needed for wcsnorm_s / mark.h. So far not needed anymore
 dnl AX_CHECK_COMPILE_FLAG([-fbracket-depth=512],
@@ -574,6 +580,8 @@ AC_CHECK_MEMBER(struct tm.tm_gmtoff,
      [#include <time.h>])
 AX_COMPILE_CHECK_SIZEOF(time_t, [#include <time.h>])
 AC_CHECK_TYPE([wchar_t],[],[],[[#include <wchar.h>]])
+AX_GCC_BUILTIN(__builtin_ctz)
+AX_GCC_BUILTIN(__builtin_object_size)
 
 AC_MSG_NOTICE([Check library functions])
 # ===============================================
@@ -601,7 +609,6 @@ AC_CHECK_FUNCS([memset strcmp strcasecmp strcasestr strcspn strpbrk strspn \
                 vsscanf_s vscanf_s printf_s fprintf_s tmpfile_s vfprintf_s vprintf_s \
                 fopen_s freopen_s gets_s bsearch_s qsort_s gmtime_s localtime_s \
                 asctime_s ctime_s getenv_s ])
-AX_GCC_BUILTIN(__builtin_ctz)
 AM_CONDITIONAL([HAVE_WCSSTR], [test "x$ac_cv_func_wcsstr" = "xyes"])
 AM_CONDITIONAL([HAVE_MBSTOWCS], [test "x$ac_cv_func_mbstowcs" = "xyes"])
 AM_CONDITIONAL([HAVE_MEMCPY_S], [test "x$ac_cv_func_memcpy_s" = "xyes"])