travis: add OSX build

travis: add OSX build

Of course OSX is not an important target but since it's available on
travis we can (mis)use it to generally improve non-Linux and non-GNU
compatibility. People are using musl, busybox or whatever and
util-linux should work for them. Having OSX build running regularly
will help us to freeze the lately improved BSD compatibility and
to check code which is usually ifdef'ed.

We are using the most recent OSX 10.11.x / Xcode 7.3 though the older
travis OSX images are working too currently.

If it turns out that this OSX build is too annoying when we are about
to add support for latest Linux features then we could add:
  allow_failures:
    - os: osx
to let it fail silently.

Note I've tried hard to keep .travis-functions.sh readable for the
Linux case. I've removed --with-python because it's default anyway,
--enable-gtk-doc is still tested in discheck.

OSX root check would also work (almost, using another --prefix) but
would only run one more test yet (minix).

OSX distcheck would have some issues with conditionally built man
pages and does not work with default distcheck confopts. No need to
fix that because distcheck is more for maintainer machines.

BTW to increase travis build speed again we could remove some targets
from the matrix without losing tested scenarios:
 - nonroot for Linux seems to be waste of time
 - clang/Linux could be root check only
For now we keep it as is.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>