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

For now, due to various factors its the only one that is supported.  In the future we should be able to support others.