Handle environment variable LLVM_COMPILER not being set slightly better.
Handle environment variable LLVM_COMPILER not being set slightly better.

travitch/whole-program-llvm,riyadparvez/whole-program-llvm,BenjaminSchubert/whole-program-llvm,travitch/whole-program-llvm,riyadparvez/whole-program-llvm,riyadparvez/whole-program-llvm,riyadparvez/whole-program-llvm,travitch/whole-program-llvm,SRI-CSL/whole-program-llvm,SRI-CSL/whole-program-llvm,BenjaminSchubert/whole-program-llvm,BenjaminSchubert/whole-program-llvm,travitch/whole-program-llvm,SRI-CSL/whole-program-llvm,SRI-CSL/whole-program-llvm,SRI-CSL/whole-program-llvm,BenjaminSchubert/whole-program-llvm