GNUInstallDirs: Apply Debian multiarch LIBDIR to more prefixes

On typical Debian amd64 system (bullseye), multiarch is setup as:

```
% cat /etc/ld.so.conf.d/x86_64-linux-gnu.conf
/usr/local/lib/x86_64-linux-gnu
/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu
```

Where:

```
% apt-cache policy libc6:amd64
libc6:
  Installed: 2.31-13+deb11u2
  Candidate: 2.31-13+deb11u2
  Version table:
 *** 2.31-13+deb11u2 500
        500 http://deb.debian.org/debian bullseye/main amd64 Packages
        100 /var/lib/dpkg/status
```

Update GNUInstallDirs to support all three cases `/`, `/usr/` and
`/usr/local/`.

Fixes: #19698

Modules/GNUInstallDirs.cmake

@@ -52,8 +52,10 @@ where ``<dir>`` is one of:
   .. versionadded:: 3.9
     run-time variable data (``LOCALSTATEDIR/run``)
 ``LIBDIR``
-  object code libraries (``lib`` or ``lib64``
-  or ``lib/<multiarch-tuple>`` on Debian)
+  object code libraries (``lib`` or ``lib64``)
+
+  On Debian, this may be ``lib/<multiarch-tuple>`` when
+  :variable:`CMAKE_INSTALL_PREFIX` is ``/``, ``/usr``, or ``/usr/local``.
 ``INCLUDEDIR``
   C header files (``include``)
 ``OLDINCLUDEDIR``
@@ -269,7 +271,9 @@ if(NOT DEFINED CMAKE_INSTALL_LIBDIR OR (_libdir_set
 
     if(__system_type_for_install STREQUAL "debian")
       if(CMAKE_LIBRARY_ARCHITECTURE)
-        if("${CMAKE_INSTALL_PREFIX}" MATCHES "^/usr/?$")
+        if("${CMAKE_INSTALL_PREFIX}" STREQUAL "/"
+            OR "${CMAKE_INSTALL_PREFIX}" MATCHES "^/usr/?$"
+            OR "${CMAKE_INSTALL_PREFIX}" MATCHES "^/usr/local/?$")
           set(_LIBDIR_DEFAULT "lib/${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'
+CMAKE_INSTALL_LIBDIR='usr/lib/arch'
 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'
+CMAKE_INSTALL_FULL_LIBDIR='/usr/lib/arch'
 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'
+CMAKE_INSTALL_LIBDIR='lib/arch'
 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'
+CMAKE_INSTALL_FULL_LIBDIR='/usr/local/lib/arch'
 CMAKE_INSTALL_FULL_LIBEXECDIR='/usr/local/libexec'
 CMAKE_INSTALL_FULL_LOCALEDIR='/usr/local/share/locale'
 CMAKE_INSTALL_FULL_LOCALSTATEDIR='/usr/local/var'