llvm-root set to incorrect path for llvm-4.0.0

[NTICompass]

When I try to install rust-1.16.0, I see:

configure: looking at LLVM
configure:
configure: not reconfiguring LLVM, external LLVM root
configure: error: program '/usr/bin/FileCheck' is missing, please install it

FileCheck is actually installed at /usr/lib/llvm/4/bin/FileCheck. The --llvm-root parameter in the ebuild (when USE=system-llvm) needs to be updated to /usr/lib/llvm/4.

We'll need to add something to the ebuild to set the path to /usr/lib/llvm/4 for llvm-4.0.0 and set to /usr for llvm-3.9 (or < 4 or whatever).

[NTICompass]

Reading through ponylang/ponyc#1592 and rust-lang/rust#37609 it seems that making rust work with llvm-4 is not an easy task...