[RFA] AIX thread support

[David Edelsohn]

>>>>> Benjamin Kosnik writes:

Ben> If you are saying that --enable-threads=aix implies that the posix threads layer is going to be 
Ben> used, then there is no reason to make a new thread header. Just reuse the 
Ben> posix (pthreads) one.

	Please look at the patch again more carefully.  AIX needs to use
threads-no.h when the GCC -pthread option is not present and
threads-posix.h when the GCC -pthread option is present.  That is exactly
why pthread is a multilib on AIX: the multilib build needs to access the
correct version of the header.

Ben> also please don't put threads stuff in _CHECK_OS, put it in _ENABLE_THREADS

	I need to put the thread stuff in _CHECK_OS because I need to
override $enable_threads for AIX.  It seems a lot worse to me to add OS
checks in _ENABLE_THREADS to override the incorrect value.

David