BUG #3728: pthread autoconf hangs

The following bug has been logged online:

Bug reference:      3728
Logged by:          heasley
Email address:      heas@shrubbery.net
PostgreSQL version: 8.1.10
Operating system:   sparc-sun-solaris2.9 gcc-4.1.1
Description:        pthread autoconf hangs
Details:

If the autoconf script ends-up with a CFLAGS that includes pthreads knobs,
for example -DREENTRANT, it will try to find pthread_join() without linking
in libpthread (ie: without gcc -pthread). The result is a conftest that
hangs because it has some half-baked stub for (among others)
pthread_create() that does nothing and thus it spins waiting for the
thread{1,2}_done variables to be set by a threads that do not exist.

squashing the result of the first check in config/acx_pthread.m4 allows it
find the proper results.

Sorry, I do not know the proper fix given all the platforms that need to
coexist.