Require targeting macOS 10.10, in case the user builds with an older SDK
or sets the target version to an older release. This shouldn't be a
problem with modern macOS SDKs, but users using or targeting older macOS
should have a better message available rather than a compile message.
The check is ugly, but it should work.
A similar check is done for block support; this should only be a problem
with users using GCC instead of clang, which is default on macOS.
Also add a message for vmnet support when printing the build config.
f0fc035087