Add posix_* weak helpers

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/compiler/compiler.h b/compiler/compiler.h
index 1a0556b..0a08bb1 100644
--- a/compiler/compiler.h
+++ b/compiler/compiler.h
@@ -15,4 +15,6 @@
 
 #define uninitialized_var(x) x = x
 
+#define __weak	__attribute__((weak))
+
 #endif