m4: fix c++ check for -f flags

Both gcc and clang generate warnings for unsupported -f$FLAG by
default, meaning that the previous implementation would consider
unsupported flags as supported (as a warning is not an error that
fails compilation).

The addition of -Werror treats warnings as errors, and will prevent
false-positives in terms of -f$FLAG support.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

m4/flags.m4

@@ -48,7 +48,7 @@ AC_DEFUN([RDC_PROG_CXX_FLAGS_IFELSE],
   CXXFLAGS_save="${CXXFLAGS}"
   as_ac_var=`echo "ac_cv_prog_cxx_flags_$1" | $as_tr_sh`
   AC_CACHE_CHECK([if $CXX accepts $1], [$as_ac_var], [
-    CXXFLAGS="${CXXFLAGS} $1"
+    CXXFLAGS="${CXXFLAGS} -Werror $1"
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [
       eval "$as_ac_var=yes"
     ],[