Don't build thread library for targets that don't support threads.

* configure.ac (pthread.h): Use AC_CHECK_HEADERS instead of
AC_CHECK_HEADER.
(target_thread_file): New.  Set from sed'ed gcc output.
(posix_threads): New.  Set from target_thread_file.  Use instead of
ac_have_pthread_h.
(pthread_create_version): Move initialization before code using it.
* configure: Regenerate.

From-SVN: r104838