Fix compiler build on non-Unix98 systems

[Eric Botcazou]

> I'm a bit concerned that SUS doesn't say that PTHREAD_MUTEX_RECURSIVE
> has to be a preprocessor macro (it might be an enum).  If there's a
> system with that property, we'll need to back this patch out and go
> for an autoconf test.

FreeBSD (5.3RC1) is affected.  Are you sure that an autoconf test is the way 
to go?  It's a target feature.  Should we create gthr-posix-norecurse.h or 
something like that, as a wrapper around gthr-posix.h using the appropriate 
preprocessor magic?  Or maybe should we just drop support for these old OSes?

-- 
Eric Botcazou