Build: Prepend, not append, PTHREAD_CFLAGS to LIBS

It shouldn't make any difference because LIBS should be empty
at that point in configure. But prepending is the correct way
because in general the libraries being added might require other
libraries that come later on the command line.

configure.ac

@@ -637,7 +637,7 @@ AS_CASE([$enable_threads],
 		echo
 		echo "POSIX threading support:"
 		AX_PTHREAD([:]) dnl We don't need the HAVE_PTHREAD macro.
-		LIBS="$LIBS $PTHREAD_LIBS"
+		LIBS="$PTHREAD_LIBS $LIBS"
 		AM_CFLAGS="$AM_CFLAGS $PTHREAD_CFLAGS"
 
 		dnl NOTE: PTHREAD_CC is ignored. It would be useful on AIX,