Specify the minimum version of the target platform (e.g. macOS or iOS)
on which the target binaries are to be deployed. CMake uses this
variable value for the
-mmacosx-version-min flag or their respective
target platform equivalents. For older Xcode versions that shipped
multiple macOS SDKs this variable also helps to choose the SDK in case
CMAKE_OSX_SYSROOT is unset.
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 should be set as a
CACHE entry (or else CMake may
remove it while initializing a cache entry of the same name).
OSX part in the variable name(s) they apply also to
other SDKs than macOS like iOS, tvOS, or watchOS.
This variable is ignored on platforms other than Apple.