[Beignet] Patches from Debian

[Rebecca N. Palmer]

Two patches from Debian's packaging of 1.0.1:

Prefer LLVM 3.5 (as recently suggested here): 
https://anonscm.debian.org/cgit/pkg-opencl/beignet.git/tree/debian/patches/llvm-search-order.patch

Don't use -D__$(USER)__ (it breaks on usernames containing special 
characters): 
https://anonscm.debian.org/cgit/pkg-opencl/beignet.git/tree/debian/patches/remove-unsafe-define-D__-USER-__.patch

Also, the following generated files are left behind after a build and 
clean, but we don't have a ready-made fix for this: 
https://anonscm.debian.org/cgit/pkg-opencl/beignet.git/tree/debian/clean