For Solaris, add -lrt correctly.

* configure.ac (LIB_SCHED_YIELD): Not avoiding defining LIB.

This fixes commit 5168b97 for Solaris linking issue. It tried to add
-lrt for build on Solaris for gpg-error, but it wrongly avoids adding
it also to LIB, which actually is needed.

Thanks also to Jörn Clausen, who submitted a report to our BTS.

• Reported-by: Tom G. Christensen <tgc@jupiterrise.com>
• GnuPG-bug-id: T1671
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>