Removed invalid clang's option

When building Embree in Debug mode with clang I got the following error: `clang: error: unknown argument: '-ftree-ter'`

I checked [clang's documentation](http://clang.llvm.org/docs/UsersManual.html) from version 3.1 up until 3.6 (skipped version 3.2) and could not find the `-ftree-ter option`, so I believe it is safe to remove this option from the build system.

Ps.: Got the error when running clang version 3.4.2-8