IOS build dir config

[hjanetzek]

This allows setting a custom IOS_BUILD_DIR in Makefile. Very handy for developing on linux and osx at the same time when osx mounts the source directory from linux - so that the build output is not written over the network.

[hjanetzek]

Yay!

[hjanetzek]

Ok no hurry for me - still figuring out how to build things for ios. So it's good that this part works for me :)

I think we could keep the default files with relative build dirs in the repo. This way it's easy to see when our changes should be committed. Only when invoking BUILD_ROOT=/... make ios-xcode the extra configure_files step is needed.

[matteblair]

That might be a good compromise. The xcuserdata file will always need to be configured, but the xcworkspacedata and pbxproj files only nee to be configured if a custom build directory is set, and in that scenario we can document that Xcode project settings won't be preserved.