Update scripts to work with XCode 6.1 on Yosemite with GitHub changes

Hello,

Following upgrading to Yosemite, XCode 6.1 and what appear to be GitHub changes, the build scripts did not work for me. The changes attached resolve my issues, and hopefully can be integrated.

Thanks,

Alex 
If ./Externals/installed is not present, I guess that testing the presence of that file is that directory would always return false, then `missing` would be set to `yes`. Is there something I'm missing?