Version independent

Should remove problems with versions of OpenFOAM earlier than 2.3.0 (I'm sure there's still a lower limit, but 2.2.2 works and I think 2.1.X should also be ok, although it's untested)

Versions of OpenFOAM newer than 2.3.0 (2.3.1 being the first example to come to mind, as yet unreleased but presumably being worked on) will break this, but only changes will be needed to the Make/options file.

These changes might be added before the release of a new OF, if I can find a good way of extending the comparisons. Alternatively, this can be hotfixed as needed by adding a new comparison which allows the setting of the MACRO_DEFINED= to blank.