Define _REENTRANT as needed on Solaris, elsewhere It turns out that _REENTRANT isn't only needed to make certain functions visible; we also need it to make pthreads work properly some places (like Solaris, where forgetting _REENTRANT basically means that all threads are sharing the same errno). Fortunately, our ACX_PTHREAD() configure macro already gives us a PTHREAD_CFLAG variable, so all we have to do is use it.