configure: Explicitly disable w32threads if the test for it fails

This avoids false positive enabling of w32threads if explicitly
requested on the command line, but dependencies are missing.

configure

@@ -3850,7 +3850,8 @@ check_header windows.h
 check_header X11/extensions/XvMClib.h
 
 if ! disabled w32threads && ! enabled pthreads; then
-    check_func_headers "windows.h process.h" _beginthreadex && enable w32threads
+    check_func_headers "windows.h process.h" _beginthreadex &&
+        enable w32threads || disable w32threads
 fi
 
 # check for some common methods of building with pthread support