Fix OS X resource fork handling (Reported-by: Richard O'Keefe)

If you happen to fetch a zip archive of the git repo and try to build
from that, you may, for example, ask erlc to build `src/._rebar.erl`.
._* are OS X resource forks and not real .erl files. This may also
happen with network filesystems on OS X. To fix that, limit the
files compiled by rebar to include only those which start with
a letter or a digit.