PR build/11881: LIBTHREAD_DB_SO can be undefined.

PR build/11881: LIBTHREAD_DB_SO can be undefined.

This patch:

 http://sourceware.org/ml/gdb-patches/2009-04/msg00115.html

Changed behaviour by only defining LIBTHREAD_DB_SO if thread_db.h exists. The
definition of LIBTHREAD_DB_SO and LIBTHREAD_DB_SEARCH_PATH should be moved
outside of #ifdef HAVE_THREAD_DB_H.

This is based on a patch attached to the PR, however, it needed a
tweak, as it was it broke the HAVE_THREAD_DB_H path.

2013-04-16  Will Newton  <will.newton@gmail.com>
	    Pedro Alves  <palves@redhat.com>

	PR build/11881

	* common/gdb_thread_db.h (LIBTHREAD_DB_SO)
	(LIBTHREAD_DB_SEARCH_PATH): Move outside of #ifdef
	HAVE_THREAD_DB_H.