cond, lock, thread: better 'inline'

cond, lock, thread: better 'inline'

* lib/glthread/cond.c, lib/glthread/cond.h (_GLTHREAD_COND_INLINE):
* lib/glthread/thread.c, lib/glthread/thread.h (_GLTHREAD_THREAD_INLINE):
New macros.  Use them instead of static inline, for header functions.
* lib/glthread/cond.c (gl_waitqueue_init, gl_waitqueue_remove)
(gl_waitqueue_notify_first, gl_waitqueue_notify_all):
* lib/glthread/lock.c (gl_waitqueue_init)
(gl_waitqueue_notify_first, gl_waitqueue_notify_all):
* lib/glthread/thread.c (get_current_thread_handle):
Change 'static inline' to 'inline'.
* lib/glthread/cond.h, lib/glthread/thread.h:
Use _GL_INLINE_HEADER_BEGIN, _GL_INLINE_HEADER_END.
* m4/cond.m4 (gl_COND):
* m4/lock.m4 (gl_PREREQ_LOCK):
* m4/thread.m4 (gl_THREAD):
Do not require AC_C_INLINE.
* modules/cond, modules/thread (Depends-on): Add extern-inline.