Use -Werror for compiler flag support checks so that they actually work

aegisub/m4macros/ac_flag.m4

 AC_DEFUN([AC_C_FLAG], [{
 	AC_LANG_PUSH(C)
 	ac_c_flag_save="$CFLAGS"
-	CFLAGS="$CFLAGS $1"
+	CFLAGS="$CFLAGS -Werror $1"
 	AC_MSG_CHECKING([[whether $CC supports $1]])
 	AC_COMPILE_IFELSE(
 		[AC_LANG_PROGRAM([[]])],
-		[AC_MSG_RESULT([yes])],
 		[
+			CFLAGS="$ac_c_flag_save $1"
+			AC_MSG_RESULT([yes])
+		], [
 			CFLAGS="$ac_c_flag_save"
 			AC_MSG_RESULT([no])
 			$2
@@ -16,11 +18,14 @@ AC_DEFUN([AC_C_FLAG], [{
 AC_DEFUN([AC_CXX_FLAG], [{
 	AC_LANG_PUSH(C++)
 	ac_cxx_flag_save="$CXXFLAGS"
-	CXXFLAGS="$CXXFLAGS $1"
+	CXXFLAGS="$CXXFLAGS -Werror $1"
 	AC_MSG_CHECKING([[whether $CXX supports $1]])
 	AC_COMPILE_IFELSE(
 		[AC_LANG_PROGRAM([[]])],
-		[AC_MSG_RESULT([yes])],
+		[
+			CXXFLAGS="$ac_cxx_flag_save $1"
+			AC_MSG_RESULT([yes])
+		],
 		[
 			CXXFLAGS="$ac_cxx_flag_save"
 			AC_MSG_RESULT([no])