Various Clang binaries are now considered when choosing the compiler for
Lagom.
The selection precedence is as follows:
1. Use the compiler set via CC/CXX if it's a supported version
2. Use newest available GCC if it's supported
3. Use newest available Clang if it's supported
Note that Apple Clang is still not supported, as its versioning scheme
and the fact that it masquerades as both GCC and Clang would complicate
this logic even more.
Fixes#12253
dbb2347000