Add support for MinGW to the Makefile system (in progress).

This is a set of updates to the makefile system to add MinGW support on windows.

This is required for ofSketch to run on Windows.  ofSketch uses the makefile system on all platforms.

It is in progress pending a few remaining issues:

https://github.com/olab-io/ofSketch/issues/49#issuecomment-51704273

http://forum.openframeworks.cc/t/windows-mingw-make-system-core-of-is-always-recompiled/16697

@bilderbuchi @bakercp, and when there is support, it could be a findstring or filter conditional that looks for MINGW32_NT in PLATFORM_OS