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

I'm not familiar with it, but that version/OS string sounds pretty specific/fragile to future changes. Does this only match for mingw 6.2, and if so, do we only support that particular version?