changed FATAL_ERROR to WARNING for llvm-config --system-libs
changed FATAL_ERROR to WARNING for llvm-config --system-libs

Newer versions of llvm (at least it's true for 4.0) appear to normally have the --system-libs option empty

Former-commit-id: 0a1a8aecb55d943cc8542d5e01a29d9e481fcb4d [formerly 534a6a4811c50c3f4fe2c8b90a9881ed53ccb7b9]
Former-commit-id: 769e3e8ef3664653e873e21fb65ded72c7542d9d
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide