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.

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