build: drop the pthread gnulib module

This was fixing a problem with old versions of mingw which had a
pthread.h that polluted the namespace with random symbols. This is no
longer relevant on our mingw platform targets.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

bootstrap.conf

@@ -79,7 +79,6 @@ pipe-posix
 pipe2
 poll
 posix-shell
-pthread
 pthread_sigmask
 recv
 regex