we no longer define the 'override' preprocessor symbol for those compilers that don't natively support the C++11 'override' keyword, as this clashes with boost.python with gcc versions prior to 4.7 (which is the first version to support the 'override' keyword). instead, we now define an 'OVERRIDE' processor symbol which either has no effect, or is a synonym for the 'override' keyword of C++11 when it is supported.
minor style tweaks.