GNUInstallDirs: Revert "Apply Debian multiarch LIBDIR to more prefixes"

Revert commit dd9584b352 (GNUInstallDirs: Apply Debian multiarch LIBDIR
to more prefixes, 2021-11-19, v3.23.0-rc1~323^2).  There are separate
problems with activating multiarch `LIBDIR` for each prefix it added:

* Prefix `/` is often used to stage an installation with `DESTDIR`
  for inclusion in a tarball package or similar.

* Prefix `/usr/local` is the default `CMAKE_INSTALL_PREFIX`, causing
  the multiarch `LIBDIR` to be cached after the first configuration,
  even if the prefix changes later.

Revert the change for now, except for the documentation update.
Further discussion will be needed to select a way to enable
multiarch `LIBDIR` for `/` and `/usr/local`.

Fixes: #23365
Issue: #19698

Modules/GNUInstallDirs.cmake

@@ -55,7 +55,7 @@ where ``<dir>`` is one of:
   object code libraries (``lib`` or ``lib64``)
 
   On Debian, this may be ``lib/<multiarch-tuple>`` when
-  :variable:`CMAKE_INSTALL_PREFIX` is ``/``, ``/usr``, or ``/usr/local``.
+  :variable:`CMAKE_INSTALL_PREFIX` is ``/usr``.
 ``INCLUDEDIR``
   C header files (``include``)
 ``OLDINCLUDEDIR``
@@ -273,9 +273,7 @@ if(NOT DEFINED CMAKE_INSTALL_LIBDIR OR (_libdir_set
 
     if(__system_type_for_install STREQUAL "debian")
       if(CMAKE_LIBRARY_ARCHITECTURE)
-        if("${CMAKE_INSTALL_PREFIX}" STREQUAL "/"
-            OR "${CMAKE_INSTALL_PREFIX}" MATCHES "^/usr/?$"
-            OR "${CMAKE_INSTALL_PREFIX}" MATCHES "^/usr/local/?$")
+        if("${CMAKE_INSTALL_PREFIX}" MATCHES "^/usr/?$")
           set(_LIBDIR_DEFAULT "lib/${CMAKE_LIBRARY_ARCHITECTURE}")
         endif()
         if(DEFINED _GNUInstallDirs_LAST_CMAKE_INSTALL_PREFIX

Tests/RunCMake/GNUInstallDirs/Root-Debian-stderr.txt

@@ -4,7 +4,7 @@ CMAKE_INSTALL_DATAROOTDIR='usr/share'
 CMAKE_INSTALL_DOCDIR='usr/share/doc/Root'
 CMAKE_INSTALL_INCLUDEDIR='usr/include'
 CMAKE_INSTALL_INFODIR='usr/share/info'
-CMAKE_INSTALL_LIBDIR='usr/lib/arch'
+CMAKE_INSTALL_LIBDIR='usr/lib'
 CMAKE_INSTALL_LIBEXECDIR='usr/libexec'
 CMAKE_INSTALL_LOCALEDIR='usr/share/locale'
 CMAKE_INSTALL_LOCALSTATEDIR='var'
@@ -19,7 +19,7 @@ CMAKE_INSTALL_FULL_DATAROOTDIR='/usr/share'
 CMAKE_INSTALL_FULL_DOCDIR='/usr/share/doc/Root'
 CMAKE_INSTALL_FULL_INCLUDEDIR='/usr/include'
 CMAKE_INSTALL_FULL_INFODIR='/usr/share/info'
-CMAKE_INSTALL_FULL_LIBDIR='/usr/lib/arch'
+CMAKE_INSTALL_FULL_LIBDIR='/usr/lib'
 CMAKE_INSTALL_FULL_LIBEXECDIR='/usr/libexec'
 CMAKE_INSTALL_FULL_LOCALEDIR='/usr/share/locale'
 CMAKE_INSTALL_FULL_LOCALSTATEDIR='/var'

Tests/RunCMake/GNUInstallDirs/UsrLocal-Debian-stderr.txt

@@ -4,7 +4,7 @@ CMAKE_INSTALL_DATAROOTDIR='share'
 CMAKE_INSTALL_DOCDIR='share/doc/UsrLocal'
 CMAKE_INSTALL_INCLUDEDIR='include'
 CMAKE_INSTALL_INFODIR='share/info'
-CMAKE_INSTALL_LIBDIR='lib/arch'
+CMAKE_INSTALL_LIBDIR='lib'
 CMAKE_INSTALL_LIBEXECDIR='libexec'
 CMAKE_INSTALL_LOCALEDIR='share/locale'
 CMAKE_INSTALL_LOCALSTATEDIR='var'
@@ -19,7 +19,7 @@ CMAKE_INSTALL_FULL_DATAROOTDIR='/usr/local/share'
 CMAKE_INSTALL_FULL_DOCDIR='/usr/local/share/doc/UsrLocal'
 CMAKE_INSTALL_FULL_INCLUDEDIR='/usr/local/include'
 CMAKE_INSTALL_FULL_INFODIR='/usr/local/share/info'
-CMAKE_INSTALL_FULL_LIBDIR='/usr/local/lib/arch'
+CMAKE_INSTALL_FULL_LIBDIR='/usr/local/lib'
 CMAKE_INSTALL_FULL_LIBEXECDIR='/usr/local/libexec'
 CMAKE_INSTALL_FULL_LOCALEDIR='/usr/local/share/locale'
 CMAKE_INSTALL_FULL_LOCALSTATEDIR='/usr/local/var'