Improve r556423 and allow building against custom LLVM version, i.e.
respect flexible LLVM_DEFAULT, by not hardcoding package version and
depending on the appropriate executable instead.

This is to help users sticking to the particular LLVM versions lower
the number of different LLVM packages installed. For example, there
is currently a mismatch between LLVM_DEFAULT assigned by the default
versions framework (90) and in this file (10; this hasn't changed).