GNUInstallDirs: Add dir argument to GNUInstallDirs_get_absolute_install_dir
Existing convention of using ${dir}
from the callers's scope is preserved if no third argument is provided. New code and the existing GNUInstallDirs usage will provide the third argument and pass the directory argument explicitly. I've added a warning if the old usage is encountered.
Regarding the warning, can we print it only when cmake_minimum_required is 3.19 or later? So we don't issue it on projects where it can't yet be fixed for compatibility reasons.
Closes: #21152 (closed)
Topic-rename: GNUInstallDirs-dir