Check for gmp when checking for mpfr

Check for gmp when checking for mpfr

There was a report on irc that the gdb check for mpfr failed when only
static libraries are available.  The issue is that mpfr depends on
gmp, but this is not handled explicitly by gdb.

Ideally upstream would switch to pkg-config.  Or even more ideally, we
would incorporate pkg-config into the compiler and not mess with any
of this.

Meanwhile, this changes gdb's configure to add gmp to the link line
when checking for mpfr.

gdb/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* configure.ac: Use gmp as a library dependency when checking for
	mpfr.