travis: Add automated OSX build

Split some of the repetitive .travis.yml logic into helper scripts, and
add cases for Mac OSX.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
buildsys: Disable annoying clang warnings

clang wants to complain about non-literal format strings
(vfprintf(stderr, fmt, ap)) and our unused batch_enc_iv array.
Make it stop.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>