Detect existence of llvm 3.7 or 3.8 dev package
Detect existence of llvm 3.7 or 3.8 dev package

Determine if the user has llvm3.7-dev or llvm3.8-dev package in Ubuntu
and configure cmake accordingly

ethcore/evmjit,debris/evmjit,ethcore/evmjit,debris/evmjit