gdb/configure: Add --with-libgmp-prefix option

This patch allows a user to tell gdb's configure script where
his GMP library is installed.

gdb/ChangeLog:

        * configure.ac: Add support for --with-libgmp-prefix.
        * Makefile.in (LIBGMP): New variable.
        (CLIBS): Include $(LIBGMP).
        * configure, config.in: Regenerate