Detect LLVM 7 without specifying binaries explicitly.
Before this commit LLVM 7 was supported, but only if one explicitly
provided LLVM_CONFIG= and CLANG= paths. As LLVM 7 is the first
version that includes our upstreamed debugging and profiling features,
and as debian is planning to default to 7 due to wider architecture
support, it seems good to support auto-detecting that version.
Author: Christoph Berg
Discussion: https://postgr.es/m/
20180912124517.GD24584@msg.df7cb.de
Backpatch: 11, where LLVM was introduced