The linker configuration file is now optional: It is loaded only if
variable CMAKE_<LANG>_USE_LINKER_CONFIGURATION is set to TRUE.
The file CMakeAddNewLanguage.txt is updated to take into account the
linker configuration.
Fixes: #26393
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.
