Require libclang 3.8

Trying to ensure distros are not packaging older libclang version. The
more recent the libclang version the better, actually.

Debian stable, Ubuntu 16.04 LTS, Fedora 24, openSUSE Leap 42.2 all have
Clang/LLVM 3.8

See:

https://repology.org/metapackage/llvm/versions