Fix test failures on hurd-i386.

- Make madvise() no-op, not implemented on Hurd.
- Skip threads tests, thread implementation problematic.

Author: sebastic

lib/File/Map.xs

@@ -135,7 +135,7 @@ static size_t page_size() {
 }
 #endif
 
-#ifdef VMS
+#if defined(VMS) || defined(__GNU__)
 #define madvise(address, length, advice) 0
 #endif
 

t/20-threads.t

@@ -5,7 +5,7 @@ use warnings;
 use Config;
 BEGIN {
 	# Yes, this is really necessary
-	if ($Config{useithreads}) {
+	if ($Config{useithreads} && $^O ne 'gnu') {
 		require threads;
 		threads->import();
 		require Test::More;