Fix MacVim by using clang.

* Use `clangStdenv`
* Remove platform specific Python `#include`
* Fix missing `MACOS_X_UNIX` define
* Bump version up
* Ignore warnings
* Don't attempt to find libs in `/usr/local`
@aespinosa Thanks! I had my `packageOverrides` set in `~/.nixpkgs/config.nix` to override the `src`, which explains why I missed this.