build: set -Werror when checking whether a -W<warning> option works

Otherwise the check is effectively a no-op and unknown options still get
turned on. This results in unknown warnings when build without
--with-extra-warnings=error:

  warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-const-variable'? [-Wunknown-warning-option]

diff --git a/m4/compiler_warnings.m4 b/m4/compiler_warnings.m4
index a1c4e12..a7d6669 100644 (file)
--- a/m4/compiler_warnings.m4
+++ b/m4/compiler_warnings.m4
@@ -56,7 +56,7 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
                      -Wpointer-arith -Winit-self \
                      -Wmissing-include-dirs -Wno-pragmas; do
                dnl GCC 4.4 does not warn when checking for -Wno-* flags (https://gcc.gnu.org/wiki/FAQ#wnowarning)
-               CFLAGS="$CFLAGS_MORE_WARNINGS $(printf '%s' "$option" | sed 's/^-Wno-/-W/')  $CFLAGS_SAVED"
+               CFLAGS="-Werror $CFLAGS_MORE_WARNINGS $(printf '%s' "$option" | sed 's/^-Wno-/-W/')  $CFLAGS_SAVED"
                AC_MSG_CHECKING([whether compiler understands $option])
                AC_TRY_COMPILE([], [],
                        has_option=yes,