The next official VVVVVV build removes 32-bit Linux (like all my other games),
and I need to get rid of the shell script on macOS at some point, so this
basically brings it up to what my other games are doing. Plus, this probably
fixes a bug where someone tries to run their executable away from the root...
Doing this is not necessary as CMake already looks up the default one
correctly and in fact breaks whenever you have multiple Xcode versions
installed so Xcode is not called Xcode.app.
(And it does not respect the default Xcode set using xcode-select)
The behavior now is to respect CMAKE_OSX_SYSROOT if set by the
user on the command line. If it's not set, try to use the hardcoded
path to the 10.9 SDK if it's present. If not, warn about the fact
that a different SDK is used.
When building on macOS targeting an older version than the version of
the SDK currently used, this prevents accidentally using APIs that are
too new (introduced in macOS versions newer than the deployment target).
