raw2dng make rules changes
Created originally on Bitbucket by daniel_fort (Daniel Fort)
Was already merged in Bitbucket before import, marked as merged by the import user
Modified make rules so raw2dng isn't built automatically when a platform is compiled. Also added switch so cross compiling raw2dng.exe on a Mac is possible.
Discussion: http://www.magiclantern.fm/forum/index.php?topic=15894.msg175296#msg175296