curl-compilers.m4: for gcc + want warnings, set gnu89 standard

To better verify that the code is C89

Closes #9542

m4/curl-compilers.m4

@@ -860,6 +860,7 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
       GNU_C)
         #
         if test "$want_warnings" = "yes"; then
+          tmp_CFLAGS="$tmp_CFLAGS -std=gnu89"
           #
           dnl Do not enable -pedantic when cross-compiling with a gcc older
           dnl than 3.0, to avoid warnings from third party system headers.