Add CATCH_VERSION_* defines for external use

I wonder how much use they will actually see, but their cost is
fairly minor.

Closes #1131

include/catch.hpp

@@ -9,6 +9,9 @@
 #ifndef TWOBLUECUBES_CATCH_HPP_INCLUDED
 #define TWOBLUECUBES_CATCH_HPP_INCLUDED
 
+#define CATCH_VERSION_MAJOR 2
+#define CATCH_VERSION_MINOR 1
+#define CATCH_VERSION_PATCH 1
 
 #ifdef __clang__
 #    pragma clang system_header