`SOURCE_ENGINE_MOCK` is part of the MM:S API and only available in 1.12
which is the dev branch of MM:S. We can build the mock SM build without
it anyway, but this is still a little gross as it is just missing loader
support which could be confusing.
This also changes AMBuildScript to first look for a checkout of MM:S
1.11 which has been the stable branch for a while, 1.10 is still allowed
as a fallback as that seems to still build without issue.
Asher Baker
2e763c287c