This removes duplicated code for per-config variable initialization by
providing a `cmake_initialize_per_config_variable(<PREFIX> <DOCSTRING>)`
function.
This function initializes a `<PREFIX>` cache variable from `<PREFIX>_INIT`
and unless the `CMAKE_NOT_USING_CONFIG_FLAGS` variable is defined, does
the same with `<PREFIX>_<CONFIG>` from `<PREFIX>_<CONFIG>_INIT` for every
`<CONFIG>` in `CMAKE_CONFIGURATION_TYPES` for multi-config generators or
`CMAKE_BUILD_TYPE` for single-config generators.
