Workaround for clang 3.2 libc++ empty struct bug.

Certain tests have empty structs or classes.
This encounters a bug with clang: http://llvm.org/bugs/show_bug.cgi?id=16764
This is fixed in later versions of clang, but not the version
currently bundled with Mavericks and XCode 5

Examples/test-suite/template_matrix.i

@@ -21,6 +21,7 @@ namespace simuPOP
   template<class _POP1, class _POP2 = POP>
   class Operator
   {
+    int x;
   };
 }