gdb: Make GMP a required dependency for building GDB

This commit modifies gdb's configure script to trigger an error
if we cannot find a usable libgmp.

For the record, making this a requirement was discussed in March 2018:
https://sourceware.org/pipermail/gdb-patches/2018-March/147373.html

gdb/ChangeLog:

        * configure.ac: Generate an error if a usable GMP library
        could not be found.
        * configure: Regenerate.