configure.in: Check for -Wno-long-long option support and improve handling of warning CFLAGS...

        * configure.in: Check for -Wno-long-long option support and improve
        handling of warning CFLAGS for stages 1 and 2.
        * Makefile.in (WARN_CFLAGS): Move definition to configure.in.
        * configure: Regenerate.

From-SVN: r34156