Support absolute paths for CMAKE_INSTALL_LIBDIR
Fixes startup for 1.2.1-5 on Artix, which sets it to /usr/lib
Draft because untested, but should work, right? tested with Artix PKGBUILD, appimage artifact, linux artifact. Just checking it opens.
https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
Edited by Nathan Lee