make_lockfile: only call posix_getpid() if it actually exists (hello, win32)

functions.php

@@ -2236,7 +2236,9 @@
 		$fp = fopen(LOCK_DIRECTORY . "/$filename", "w");
 
 		if (flock($fp, LOCK_EX | LOCK_NB)) {
-			fwrite($fp, posix_getpid() . "\n");
+			if (function_exists('posix_getpid')) {
+				fwrite($fp, posix_getpid() . "\n");
+			}
 			return $fp;
 		} else {
 			return false;