suppressing -Wundef in clang (GTest emitted)

tests/notnull_tests.cpp

@@ -26,6 +26,7 @@
 #if __clang__ || __GNUC__
 //disable warnings from gtest
 #pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wundef"
 #pragma GCC diagnostic ignored "-Wglobal-constructors"
 #pragma GCC diagnostic ignored "-Wused-but-marked-unused"
 #pragma GCC diagnostic ignored "-Wcovered-switch-default"