OS X's setup.command in buildenv/1.3.x/osx/setup.command should have a terminal-friendly mode

[mkrautz]

Instead of opening the Finder, it should detect if it's being run from a terminal directly, and source the 'env' file and put you in the buildenv -- it should also cd into ${MUMBLE_PREFIX}.

(This report also applies to buildenv/1.2.x, as well as the osx-universal variants...)

Note: this doesn't break CI, as CI uses ./setup.command --non-interactive (which just prints the BUILDENV_TARGET (which is equivalent to MUMBLE_PREFIX).

[mkrautz]

Not really a big problem.