Custom string versions for compilers were raising a ValueError on
conversion to int. This commit fixes the behavior by trying to detect
the underlying compiler version when in presence of a custom string
version.
* Refactor code that deals with custom versions for better readability
* Partition version components with a regex
* Fix semantic of custom compiler versions with a suffix
* clang@x.y-apple has been special-cased
* Add unit tests
498f448ef3