[cfe-dev] mingw search path recommendation

[Nikola Smiljanic]

I don't think any changes will make it for the 3.1 release, at least
not from my side. I just started poking around the driver as Chandler
suggested. The previous patch was my attempt to fix the broken mingw
search paths (two missing C include directories) but I'm not an expert
when it comes to mingw. I'm not sure if this new logic will cover
Cygwin/MSYS or if they should be handled like real GCC.