[binutils-gdb] Use AM_ENABLE_MULTILIB only if with_target_subdir isn't empty

[sergiodj+buildbot@redhat.com]

*** TEST RESULTS FOR COMMIT f85997a697553fb7053de89bafe36e3d71c6f784 ***

Author: H.J. Lu <hjl.tools@gmail.com>
Branch: master
Commit: f85997a697553fb7053de89bafe36e3d71c6f784

Use AM_ENABLE_MULTILIB only if with_target_subdir isn't empty
	* configure.ac (AM_ENABLE_MULTILIB): Use only if
	${with_target_subdir} isn't empty.
	* configure: Regenerated.