travis: test "make install" and "make dist" on macOS

travis: test "make install" and "make dist" on macOS

We can't use "make distcheck" on macOS because many unit tests fail. We
can still get coverage of some of the things "distcheck" validates, by
running the "install" and "dist" targets. This is particularly useful
because many conditional features are disabled on macOS, and this helps
make sure we can still successfully install & dist when these bits are
disabled.

The default script is getting unreadable since it is all on one long
line. Rather than adding further conditional clauses to it, we make
use of the travis matrix config override for the script.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>