threads: Fix building with POSIX threads in MinGW.

BUG: https://red.libssh.org/issues/181

Originally written by Patrick von Reth <vonreth () kde ! org>.

This patch is part of the larger patch:
https://projects.kde.org/projects/kdesupport/emerge/repository/revisions/master/changes/portage/win32libs/libssh/0002-add-a-way-to-test-ssh-connections-on-windows.patch

MinGW (in particular, the MinGW-w64 fork) can use either posix threads
or win32 threads. This patch fixes the MinGW build when using posix
threads.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 433f8fd550)

src/threads/pthread.c

@@ -75,7 +75,7 @@ static int ssh_pthread_mutex_unlock (void **lock){
 }
 
 static unsigned long ssh_pthread_thread_id (void){
-#if _WIN32
+#if defined(_WIN32) && !defined(__WINPTHREADS_VERSION)
     return (unsigned long) pthread_self().p;
 #else
     return (unsigned long) pthread_self();