kernel: search in `pwd`/kernel/ for kernels.

Closes https://github.com/veox/sgminer/issues/49.
Merge remote-tracking branch 'tonokip/master'
build/mingw: style fixup after tonobitc merge.

Use `#defined(...)` instead of `#defined ...`.

Remove comments about issues - anyone removing those lines can check blame
to see why they were added.

Add tonobitc to AUTHORS.md.