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>
buildsys: Fix missing stc-*.c file in "make dist"

By default, automake only packages stc-@CRYPTO_BACKEND@.c.  But we want
to ship both variants.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
buildsys: Don't add -ltomcrypt to "pkg-config --libs" after fallback to nettle

If we used nettle because libtomcrypt was unavailable (not because the
user explicitly asked for nettle), @EXTRA_PC_LIBS@ was incorrectly
getting set to "-ltomcrypt".  Fix the logic so this does not happen.

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