Add CMAKE_INSTALL_CMAKEBASEDIR variable to GNUInstallDirs. This variable can be used to specify the base location for the installation of CMake configuration files. The complete installation location can be obtained by appending to CMAKE_INSTALL_CMAKEBASEDIR the PACKAGE_NAME of the installed package.
Fixes: #18381 (closed)
Topic-rename: GNUInstallDirs-CMAKEBASEDIR