GNUInutallDirs: Suppress LIBDIR warning for LANGUAGES NONE projects

Projects that explicitly use LANGUAGES NONE, e.g. to install only
data files, have no target architecture, which is intentional.
Default to `lib` without warning.

Fixes: #23461

Modules/GNUInstallDirs.cmake

@@ -90,6 +90,10 @@ where ``<dir>`` is one of:
   :variable:`CMAKE_INSTALL_PREFIX` is ``/usr``.
   When cross-compiling, the default is ``lib``.
 
+  In projects that enable no languages, e.g.,
+  ``project(... LANGUAGES NONE)``, no target architecture information
+  is available, so the default is ``lib``.
+
   .. note::
 
     When an alternative installation prefix is specified with the
@@ -444,10 +448,13 @@ set(_GNUInstallDirs_DATAROOTDIR_DEFAULT "share")
 function(_GNUInstallDirs_LIBDIR_get_default out_var install_prefix)
   set(${out_var} "${_GNUInstallDirs_LIBDIR_DEFAULT}")
 
-  if (NOT DEFINED CMAKE_SYSTEM_NAME OR NOT DEFINED CMAKE_SIZEOF_VOID_P)
-    message(AUTHOR_WARNING
-      "Unable to determine default CMAKE_INSTALL_LIBDIR directory because no target architecture is known. "
-      "Please enable at least one language before including GNUInstallDirs.")
+  if(NOT DEFINED CMAKE_SYSTEM_NAME)
+    message(AUTHOR_WARNING "Unable to determine default CMAKE_INSTALL_LIBDIR directory because no target platform is known.")
+  elseif(NOT DEFINED CMAKE_SIZEOF_VOID_P)
+    get_property(_enabled_languages GLOBAL PROPERTY ENABLED_LANGUAGES)
+    if(NOT _enabled_languages STREQUAL "NONE")
+      message(AUTHOR_WARNING "Unable to determine default CMAKE_INSTALL_LIBDIR directory because no target architecture is known.")
+    endif()
   endif()
 
   # When not cross-compiling, detect the host distro's arch-specific 'lib'.

Tests/RunCMake/GNUInstallDirs/NoArch-stderr.txt

@@ -0,0 +1,10 @@
+^CMake Warning \(author\) at .*/Modules/GNUInstallDirs\.cmake:[0-9]+ \(message\):
+  Unable to determine default CMAKE_INSTALL_LIBDIR directory because no
+  target architecture is known\.
+Call Stack \(most recent call first\):
+  .*GNUInstallDirs\.cmake:[0-9]+ \(_GNUInstallDirs_LIBDIR_get_default\)
+  .*GNUInstallDirs\.cmake:[0-9]+ \(cmake_language\)
+  .*GNUInstallDirs\.cmake:[0-9]+ \(_GNUInstallDirs_cache_path\)
+  NoArch\.cmake:[0-9]+ \(include\)
+  CMakeLists\.txt:[0-9]+ \(include\)
+This warning is for project developers\.  Use -Wno-author to suppress it\.$

Tests/RunCMake/GNUInstallDirs/NoArch.cmake

@@ -0,0 +1,3 @@
+enable_language(C)
+unset(CMAKE_SIZEOF_VOID_P)
+include(GNUInstallDirs)

Tests/RunCMake/GNUInstallDirs/NoLanguages.cmake

@@ -0,0 +1 @@
+include(GNUInstallDirs)

Tests/RunCMake/GNUInstallDirs/NoSystem-stderr.txt

@@ -1,7 +1,6 @@
 ^CMake Warning \(author\) at .*/Modules/GNUInstallDirs\.cmake:[0-9]+ \(message\):
   Unable to determine default CMAKE_INSTALL_LIBDIR directory because no
-  target architecture is known\.  Please enable at least one language before
-  including GNUInstallDirs\.
+  target platform is known\.
 Call Stack \(most recent call first\):
   .*GNUInstallDirs\.cmake:[0-9]+ \(_GNUInstallDirs_LIBDIR_get_default\)
   .*GNUInstallDirs\.cmake:[0-9]+ \(cmake_language\)

Tests/RunCMake/GNUInstallDirs/RunCMakeTest.cmake

@@ -38,6 +38,8 @@ block()
 endblock()
 
 run_cmake(GetAbs)
+run_cmake(NoArch)
+run_cmake(NoLanguages)
 run_cmake(NoSystem)
 
 foreach(case