Fix gdbserver-without-gdb build

An earlier patch changed gdbserver to use the already-built top-level
gnulib and gdbsupport.  However, if one did a build that did not
include gdb, then gdbserver would fail to build.

The problem is that configure.ac only adds gnulib and gdbsupport to
the build when gdb is being built.  This patch fixes the problem by
arranging for this to happen when gdbserver is built.

ChangeLog
2020-02-17  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* configure.ac (configdirs): Add gnulib and gdbsupport when building
	gdbserver.