config.if (cxx_interface, [...]): Do not try to set these if the appropriate directories and files to not exist.

P
        * config.if (cxx_interface, libstdcxx_interface): Do not try to set
        these if the appropriate directories and files to not exist.

From-SVN: r23166

        * configure (gxx_include_dir): Changed to
        '${prefix}/include/g++'-${libstdcxx_interface}.
        * config.if: New to determine the interfaces.

From-SVN: r22109