Support for overriding CC, CXX, CFLAGS, CXXFLAGS from a config file

[crlf0710]

I'm a windows user, and i've got mingw64 installed. Either add mingw64 dirs into PATH or setting CC CXX etc i can get things compiled. However i prefer not to do so, because i've got msys2 toolchains and msvc toolchains too. I'd rather keep things separated instead of messing them together.

I wonder if gcc-rs can load these from a separate overriding config file (only used by itself), so i don't have to tweak the whole system environment.

[alexcrichton]

Thanks for the report! I think that this functionality is covered by #82, so I'm going to close in favor of that.

I'm still in favor of doing though!