Allow for preprocessor definitions in blocks

[chaoticbob]

Some blocks need to have preprocessor definitions for dependencies, it would be nice to have a feature that allows adding custom preprocessor definitions into the generated projects. There should also be a way to separate this out per platform.

[richardeakin]

Just noting that this does seem to be implemented, using the <preprocessorDefine> element like here.

Also noting that while I see it in my project's Preprocessor Defintions, it is in a little bit of an odd form - defined differently for each configuration. So If you are selecting "All Configurations" then the defines are stuck behind the <different options> glob. My guess is that these extra definitions were added after the _DEBUG=1 that gets defined for Debug configuration and NDEBUG for Release - if they were somehow added before that then I imagine VS would all you to edit them in the IDE when you have 'All Configurations' selected.