Fix __must_check compile on older compilers

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/compiler-gcc3.h b/compiler-gcc3.h
new file mode 100644
index 0000000..ab8ccc4
--- /dev/null
+++ b/compiler-gcc3.h
@@ -0,0 +1,8 @@
+#ifndef FIO_COMPILER_GCC3_H
+#define FIO_COMPILER_GCC3_H
+
+#if __GNUC_MINOR__ >= 4
+#define __must_check		__attribute__((warn_unused_result))
+#endif
+
+#endif