Add a --enable-osx-deploy-target option to set the deployment target

Add a --enable-osx-deploy-target option to set the deployment target
when building for OS X; that causes the MACOSX_DEPLOYMENT_TARGET
environment variable to be set when building (so that, for example, we
don't use linker features available on the version on which we're
building but not on the minimum OS version for which we're building),
and causes the SDK for that version to be used (so that, for example, we
don't link with libraries with later version numbers than the ones
provided with the OS version for which we're building).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@50410 f5534014-38df-0310-8fa8-9805f1628bb7