Revert: Don't enable gdbtk in testsuite

Revert: Don't enable gdbtk in testsuite

This patch is to revert my previous commit, because we shouldn't remove
gdbtk bits from gdb/testsuite/configure.ac while keep gdbtk bits in
gdb/configure.ac.

gdb/testsuite:

2014-12-05  Yao Qi  <yao@codesourcery.com>

	Revert:

	* configure.ac: Remove AC_ARG_ENABLE for gdbtk.  Don't invoke
	AC_CONFIG_SUBDIRS(gdb.gdbtk).
	* configure: Re-generated.