When testing the stderr produced by various thread-support flags, also
run a compilation in addition to a link, because clang warns on
certain flags when compiling but not when linking.

will use pthreadGC2.

Dave Page

compiler emits any warnings, the test program had better be 100%
correct, not only 90% correct.  The recent addition of -Wold-style-definition
broke thread-safety detection on every platform that has that switch,
because the test program used an old-style definition.

than /dev/null, which Win32 doesn't have.

test only tests for building a binary, not building a shared library.

On Linux, you can build a binary with -pthread, but you can't build a
binary that uses a threaded shared library unless you also use -pthread
when building the binary, or adding -lpthread to the shared library
build.  This patch has the effect of doing the later by adding both
-pthread and -lpthread when building libpq.

	-D_REENTRANT -D_THREAD_SAFE -D_POSIX_PTHREAD_SEMANTICS

for all ports.  It can't hurt if they are not supported, but it makes
our job easier for porting.

Should fix Darwin compile and other platforms without mucking with the
thread detection code.

Allow additional thread flags to be added via port templates.

Change thread flag names to PTHREAD_CFLAGS and PTHREAD_LIBS to match new
configure script.