expat: Fix linking with system Expat via its CMake package

The Expat package intentionally provides the lowercase imported target
name `expat::expat` [1].  CMake performs `find_package(EXPAT)` w/o
specifying a finding mode (CONFIG or MODULE).  However, if a user
provides `CMAKE_FIND_PACKAGE_PREFER_CONFIG=TRUE`, the build fails due to
an unknown `EXPAT::EXPAT` target.

[1] https://github.com/libexpat/libexpat/tree/R_2_7_4/expat#using-libexpat-in-your-cmake-based-project

Issue: #27646

Source/Modules/CMakeBuildUtilities.cmake

@@ -181,6 +181,10 @@ if(CMAKE_USE_SYSTEM_EXPAT)
   endif()
   set(CMAKE_EXPAT_INCLUDES ${EXPAT_INCLUDE_DIRS})
   set(CMAKE_EXPAT_LIBRARIES ${EXPAT_LIBRARIES})
+  # Expat's CMake package provides a lower-case target name.
+  if(NOT TARGET EXPAT::EXPAT AND TARGET expat::expat)
+    add_library(EXPAT::EXPAT ALIAS expat::expat)
+  endif()
 else()
   set(CMAKE_EXPAT_INCLUDES)
   set(CMAKE_EXPAT_LIBRARIES cmexpat)