For llvm8-dev llvm-config executable is no longer in PATH
(it is in PATH now for llvm9-dev) and now it is in
/usr/lib/llvm8/bin/llvm-config, so it is more reliable
to adjust $PATH before calling meson build.
It will work for all past, current and future llvm versions.
Fixes 90d3deb7b4
[ci:skip-build]: already built successfully in CI