Fix for issues #12, #18: login folder includes; test_mips compilation

I didn't totally understand your comment on #12, but it doesn't work for me. Needed to add the login folder to the include path in order for `#include`s from any within that subdirectory to be relative to `src/$(LOGIN)`.

Also, the compilation of `test_mips` omits files of the form `test_mips_*.c[pp]`, specified as allowable.