[Vendor] Use `MACOSX_DEPLOYMENT_TARGET` when building for OS X.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-412
[iOS Config] Retrieve DTSDKBuild & DTPlatformBuild programatically.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-415.
[Vendor] BS files should also go in the platform specific build dir.

* Because they can be different per platform.
* Because this way they are deleted as part of `rake clean.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-416
2.21