aclocal.m4 (GCC_HEADER_STRING): New macro to detect if it is safe to include both string.h and strings.h together.

        * aclocal.m4 (GCC_HEADER_STRING): New macro to detect if it is
        safe to include both string.h and strings.h together.
        (GCC_NEED_DECLARATION): Test STRING_WITH_STRINGS when deciding
        which headers to search for function declarations.  Continue to
        prefer string.h over strings.h when both are not acceptable.
        * acconfig.h (STRING_WITH_STRINGS): Add stub.
        * configure.in: Call GCC_HEADER_STRING.
        * system.h: Test STRING_WITH_STRINGS when deciding which headers
        to include.  Continue to prefer string.h over strings.h when both
        are not acceptable.

From-SVN: r22204