configure.in (USE_UNWIND_EXCEPTIONS): Add support to set USE_UNWIND_EXCEPTIONS if --enable-libunwind-exceptions is set.

        * configure.in (USE_UNWIND_EXCEPTIONS): Add support to set
        USE_UNWIND_EXCEPTIONS if --enable-libunwind-exceptions is set.
        * configure, config.in: Regenerate.

        * g++spec.c (LIBUNWIND): New.
        (lang_specific_driver): Add it if USE_UNWIND_EXCEPTIONS is set.

From-SVN: r54636