between shiboken2's build system picking the highest versioned
llvm-config available and pkg registering a dependency on the
highest versioned libclang.so available, we need to
- pass the correct llvm-config via cmake variable and not via
  environment, as the environment variable is not used by
  shiboken's build system
- specify the dependency on libclang.so with it's version number; and
  to avoid breaking (or requiring manual intervention) with the next
  LLVM_DEFAULT switch, extract that version number from LLVM_DEFAULT
  in a way that should be working for llvm versions from 6 to 59
  (unless we choose to change our llvm port naming scheme), which
  seems to give us some safety margin.

PR:		253749
Approved by:	lbartoletti (kde@)