m4: prepare gl_WARN_ADD for simultaneous use of multiple compilers

m4: prepare gl_WARN_ADD for simultaneous use of multiple compilers

* m4/warnings.m4 (gl_COMPILER_OPTION_IF): Change gl_Flags to use
_AC_LANG_PREFIX[]FLAGS in unquoted form.  Change gl_Warn to use gl_Flags
instead of _AC_LANG_ABBREV.  Change notification message to mention
the variable name of the compiler.
(gl_UNKNOWN_WARNINGS_ARE_ERRORS): Do nothing if
gl_unknown_warnings_are_errors is set.
(gl_WARN_ADD): Change the use of gl_UNKNOWN_WARNINGS_ARE_ERRORS from
AC_REQUIRE to a regular call.