detect LLVM installed

[ponyatov]

make runs local LLVM 10 build, it would be better to use already installed LLVM, at least check and warn that it must be installed manually.

It is important because LLVM build is resource and time-hungry, and that should be done once, and the best way is using the libllvm-dev already provided by host system distribution.

[Araq]

But then nlvm cannot ensure the quality of the outcome. If nlvm is tested with LLVM 10, it should build with LLVM 10, not with LLVM X just because a "build is resource and time-hungry".

[arnetheduck]

I'm happy to take a patch that allows the caller of make to be adventurous with their llvm versions and provide a different one as an option (using an env or make parameter), but generally, nlvm needs a precise version with precise compile options - also, every distro out there is different, so the patch I'd accept would have readme instructions specific to the distro.