Guard __must_check ifdef

Some systems seem to pick up the kernel definitions, so check
if __must_check is defined before (potentially) redefining it.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/compiler/compiler-gcc3.h b/compiler/compiler-gcc3.h
index ab8ccc4..566987a 100644
--- a/compiler/compiler-gcc3.h
+++ b/compiler/compiler-gcc3.h
@@ -2,7 +2,9 @@
 #define FIO_COMPILER_GCC3_H
 
 #if __GNUC_MINOR__ >= 4
+#ifndef __must_check
 #define __must_check		__attribute__((warn_unused_result))
 #endif
+#endif
 
 #endif