I've found the problem. It looks like the
pthreads-win32 coders already know about it, as
it is commented in the set_getvalue.c source file:


      /* Note:
       *  The windows NT documentation says that the increment must be
       *  greater than zero, but it is set to zero here. If this works,
       *  the function will return true. If not, we can't do it this way
       *  so flag it as not implemented.
       */

It has to do with the differences between the Win98
and Win NT implementation of the function 
ReleaseSemaphore(). For the record, I ran the same piece of 
code on 98 and NT platforms, and the sem_getvalue() function
does *not* work on NT (*as was suspected).

Thanks again everyone
Rob