CMakePackageConfigHelpers: only emit arch check if needed

Before this patch, the write_basic_package_version_file() function of the CMakePackageConfigHelpers module always emitted an architecture check, even if the ARCH_INDEPENDENT option was specified. While this is not an issue when configuring builds, as the check is skipped, this can create issues when the "arch independent" version files are installed in the datadir (e.g. /usr/share) in a MultiArch environment like Debian, where different architecture packages of the same libraries can be coinstalled; as the amd64 version of a given library contains "8 * 8" in the file, while the i386 one contains "4 * 8", there's a conflict, as files in /usr/share are expected to be identical across architectures.

This patch fixes this issue by only emitting the architecture check code if needed; when ARCH_INDEPENDENT is specified, no code is written at all.

Here's a diff between the version files generated before and after this patch:

diff -u old/indep.cmake new/indep.cmake
--- old/indep.cmake	2023-01-29 13:43:04.840671117 +0100
+++ new/indep.cmake	2023-01-29 13:57:28.475191551 +0100
@@ -52,19 +52,3 @@
 endif()
 
 
-# if the installed project requested no architecture check, don't perform the check
-if("TRUE")
-  return()
-endif()
-
-# if the installed or the using project don't have CMAKE_SIZEOF_VOID_P set, ignore it:
-if("${CMAKE_SIZEOF_VOID_P}" STREQUAL "" OR "8" STREQUAL "")
-  return()
-endif()
-
-# check that the installed version has the same 32/64bit-ness as the one which is currently searching:
-if(NOT CMAKE_SIZEOF_VOID_P STREQUAL "8")
-  math(EXPR installedBits "8 * 8")
-  set(PACKAGE_VERSION "${PACKAGE_VERSION} (${installedBits}bit)")
-  set(PACKAGE_VERSION_UNSUITABLE TRUE)
-endif()
diff -u old/no-indep.cmake new/no-indep.cmake
--- old/no-indep.cmake	2023-01-29 13:42:05.010710508 +0100
+++ new/no-indep.cmake	2023-01-29 13:57:40.914237219 +0100
@@ -52,13 +52,8 @@
 endif()
 
 
-# if the installed project requested no architecture check, don't perform the check
-if("FALSE")
-  return()
-endif()
-
 # if the installed or the using project don't have CMAKE_SIZEOF_VOID_P set, ignore it:
-if("${CMAKE_SIZEOF_VOID_P}" STREQUAL "" OR "8" STREQUAL "")
+if(CMAKE_SIZEOF_VOID_P STREQUAL "" OR "8" STREQUAL "")
   return()
 endif()
 

Fixes: #24375 (closed)
Topic-rename: CMakePackageConfigHelpers-ARCH_INDEPENDENT