WCDH: Make the header -Wundef safe for the C language.

The __STDC_VERSION__ macro may be defined or not depending on the
implementation dialect of C.  Test that it is defined before testing
its value.

The CXX tests do not need such a change because they define __cplusplus
in all dialects.

Tests/Module/WriteCompilerDetectionHeader/c_undefined.c

@@ -0,0 +1,7 @@
+
+#include "test_compiler_detection.h"
+
+int main()
+{
+  return 0;
+}