build-sys: Append -Werror when testing flags

Clang don't treat unknown warnings flags as an error, but rather as a
warning. The result is that the detection for whic CFLAGS are supported
by this compiler will not work, since the compilation will succeed.

With this patch we now successfully detect clang doesn't support
-Wlogical-op, as opposed to previous behavior:

	checking if clang supports flag -Wlogical-op in envvar CFLAGS... no

We use this macro only for LDFLAGS and CFLAGS, so it's safe to stash
-Werror there.

diff --git a/m4/attributes.m4 b/m4/attributes.m4
index e354375e349a385b7b8e70dd29f10a5ba2d6abe3..f0bcf2421190af6d2a80529f45f1d9068bafaa42 100644 (file)
--- a/m4/attributes.m4
+++ b/m4/attributes.m4
@@ -41,7 +41,7 @@ AC_DEFUN([CC_CHECK_FLAG_APPEND], [
   AC_CACHE_CHECK([if $CC supports flag $3 in envvar $2],
                  AS_TR_SH([cc_cv_$2_$3]),
                 [eval "AS_TR_SH([cc_save_$2])='${$2}'"
-                 eval "AS_TR_SH([$2])='$3'"
+                 eval "AS_TR_SH([$2])='-Werror $3'"
                  AC_COMPILE_IFELSE([AC_LANG_SOURCE([int a = 0; int main(void) { return a; } ])],
                                     [eval "AS_TR_SH([cc_cv_$2_$3])='yes'"],
                                     [eval "AS_TR_SH([cc_cv_$2_$3])='no'"])