Fix makefile portability 

Improve compatibility with mingw
> The problem is that mkdir returns a failed exit state if it tries to re-create an already existing directory

Do we care if the directory already exists? I assume if the directory exists we can just continue on as normal correct?

> And there doesn't seem to be an easy portable way to test for existing directories!

Assume the above is correct I think you can do the following `@$(MKDIR) lib || true`. This will just will suppress the error that the direct already (and any other errors unfortunately).