Define _REENTRANT as needed on Solaris, elsewhere

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.