devel/shiboken2: depend on correct llvm port

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@)