warnings, manywarnings: Add support for multiple languages, not just C.
* warnings.m4 (gl_UNKNOWN_WARNINGS_ARE_ERRORS_IMPL): Renamed from
gl_UNKNOWN_WARNINGS_ARE_ERRORS.
(gl_UNKNOWN_WARNINGS_ARE_ERRORS(C)): New macro.
(gl_UNKNOWN_WARNINGS_ARE_ERRORS(C++)): New macro.
(gl_UNKNOWN_WARNINGS_ARE_ERRORS): Dispatch to
gl_UNKNOWN_WARNINGS_ARE_ERRORS(_AC_LANG).
(gl_WARN_ADD): Require the gl_UNKNOWN_WARNINGS_ARE_ERRORS specialization
of the current language. If C++ is the current language, modify
WARN_CXXFLAGS instead of WARN_CFLAGS.
* manywarnings.m4 (gl_MANYWARN_ALL_GCC(C)): New macro, extracted from
gl_MANYWARN_ALL_GCC.
(gl_MANYWARN_ALL_GCC): Dispatch to gl_MANYWARN_ALL_GCC(_AC_LANG).