Specify the minimum version of OS X on which the target binaries are
to be deployed. CMake uses this value for the
flag and to help choose the default SDK
If not set explicitly the value is initialized by the
MACOSX_DEPLOYMENT_TARGET environment variable, if set,
and otherwise computed based on the host platform.
The value of this variable should be set prior to the first
enable_language() command invocation
because it may influence configuration of the toolchain and flags.
It is intended to be set locally by the user creating a build tree.
This variable is ignored on platforms other than OS X.