Unhardcode the CMAKE_CONFIGURATION_TYPES values

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.

Modules/CMakeGenericSystem.cmake

@@ -1,6 +1,7 @@
 # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
 # file Copyright.txt or https://cmake.org/licensing for details.
 
+include(CMakeInitializeConfigs)
 
 set(CMAKE_SHARED_LIBRARY_C_FLAGS "")            # -pic
 set(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-shared")       # -shared