configure: remove dependencies on gmp and mpfr when gdb is disabled

Since 99118062, the configure checks
about GMP and MPFR for gdb builds have been moved to the toplevel
configure.
However, it doesn't take into account the --disable-gdb option. Meaning
that a build without gdb will require these libraries even if not
needed.

ChangeLog:

	* configure.ac: Skip GMP and MPFR when --disable-gdb is
	provided.
	* configure: Regenerate.