Support for setting Deployment Target and Base SDK

[ssisak (Steve Sisak)]

I'm using macports to build static libraries (i.e. ffmpeg and its dependencies) which will eventually be linked into an embedded framework in an application that may be deployed on multiple platforms, including systems older than the one I'm building on. In this case I'd like to build on 10.12 and deploy on 10.9 or later.

In Xcode, I'd just set MACOSX_DEPLOYMENT_TARGET=10.9 and be done.

It appears that macports had a similar macports.conf feature (universal_target) which was removed, linking back to r30396

Looking for a way to set this so that I don't get errors linking static libraries built with macorts into a framework with a different deployment target for instance:

ld: warning: object file (/Users/sgs/GotoReplay/Source/gotoreplay/Common/IOXMediaFoundation/FFmpeg/local/lib/libavutil.a(dict.o)) was built for newer OSX version (10.12) than being linked (10.9)

There are also valid reasons to set SDKROOT

[ryandesign (Ryan Carsten Schmidt)]

You can set macosx_deployment_target and macosx_sdk_version in macports.conf. They're undocumented because not all ports honor these settings.