Specify the minimum version of OS X on which the target binaries are to be deployed. CMake uses this value for the -mmacosx-version-min flag and to help choose the default SDK (see CMAKE_OSX_SYSROOT).
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 project() or 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.
