[libgomp] Add Unix95 mutex support

[Rainer Orth]

Jakub Jelinek writes:

> While we talk about Tru64, we should really get rid of the bogus
> /* Some implementations of <pthread.h> require this to be defined.  */
> #ifndef _REENTRANT
> #define _REENTRANT 1
> #endif
> that was added to gthr-posix.h to make Tru64 happy.  If Tru64 needs
> something like that, it needs to do that in its own headers, not generic
> ones used on all arches.  With this hunk, code that wants to see if
> -pthread was or was not used on command line suddenly can't find that out,
> as any code that through C++ headers includes gthr-posix.h will define
> _REENTRANT, no matter if -pthread was used or not.

I know, and once had a patch to do just that.  I've been very busy for a
long time with no spare time left for any GCC work until recently.  I hope
to resurrect that patch soon, but am currently working on making sure 4.1
works on my platforms.

	Rainer

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Rainer Orth, Faculty of Technology, Bielefeld University