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 
Script incorrectly determined that files were not missing, when in fact they were. ../Externals/installed was not present.