Can not use `CMAKE_OSX_DEPLOYMENT_TARGET` because it will set the
minimum version for macosx and not iphoneos or ios-simulator. Issue filed
in CMake: cmake/cmake#17431

Instead, update the proper flags in the toolchain files by passing the
`IOS_DEVELOPMENT_TARGET` during configuration.

Default `IOS_DEVELOPMENT_TARGET` to the current sdk version (latest). It
used to default to ios 5.0 for the simulators (2011) and ios 8.0 for the
devices (2014).