Fix issue #10155 - default value of CMAKE_OSX_DEPLOYMENT_TARGET should always be the empty string. When the value of CMAKE_OSX_DEPLOYMENT_TARGET is the empty string, the -mmacosx-version-min flag should not show up on the compiler command line. The logic for selecting default value of CMAKE_OSX_SYSROOT is orthogonal to and independent of the value of the deployment target. The default value for CMAKE_OSX_SYSROOT is the SDK that corresponds to the current version of Mac OSX on which cmake is running.