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

was this tested on VS - can VS handle forward slashes okay?