Add an OSX_DEPLOY_TARGET option to control building for a target OS version.

It should be set to an OS version, such as "10.5" or "10.6" or..., and,
if set (and if you're building for "APPLE"), it'll try to find the SDK
(and fail if it's not found), and add appropriate -isysroot,
-mmacosx-version-min, and, if appropriate, -m32 compiler options.

(It will need more work once we support building OS X app bundles and
installers; pick up more stuff from configure.ac for that.)

Change-Id: I0aebf2db8aa39304a41be4e2a9879dea6f308ec7
Reviewed-on: https://code.wireshark.org/review/3388
Reviewed-by: Guy Harris <guy@alum.mit.edu>