Fix compile failure on systems not supporting gthreads.

Fix compile failure on systems not supporting gthreads.

I just realized that it is possible to run a check by #undef - ing
 __GTHREADS_CXX0X in async.h.  Doing so promptly found another syntax
 error, which this version of the patch fixes.

PR libfortran/123446
PR libfortran/119136

libgfortran/ChangeLog:

	* io/async.h: DEBUG_ASYNC needs gtreads support.
	(LOCK_UNIT): Only lock when there is pthreads support and it is active.
	Otherwise, just set unit->self to 1.
	(UNLOCK_UNIT): Only unlock when there is pthreads support and it is active.
	Otherwise, just set unit->self to 0.
	(TRYLOCK_UNIT): Only try locking when thee is pthreads support and it is
	active.  Otherwise, return unit->self.
	(OWN_THREAD_ID): New macro.
	* io/io.h: gfc_unit's self is an int when there is no gthreads support.
	* io/unit.c (check_for_recursive): Check for equality of unit which
	locked to OWN_THREAD_ID.