GNUInstallDirs: Fix CMAKE_INSTALL_LIBDIR on Debian

When not installing in /usr (i.e. /usr/local) on Debian/Ubuntu, the
multiarch path is not in ldconfig path, and therefore libraries cannot
be found, unless the user manually modifies the files in
/etc/ld.so.conf.d, in order to include /usr/local/lib/<arch>.

This patch fixes this issue by using "lib/<arch>" only if the user is
installing the software in /usr, and uses "lib" in all the other cases.
Merge topic 'GNUInstallDirs_debian-multiarch-fix' into next

620939e4 GNUInstallDirs: Fix CMAKE_INSTALL_LIBDIR on Debian