CMake: Prepare dedicated license file for our own CPack configuration

The CPack WIX Generator requires a `.txt` file extension.
Provide that regardless of the actual license file extension.

CMakeCPack.cmake

@@ -14,7 +14,9 @@ if(CMake_INSTALL_DEPENDENCIES)
   include(${CMake_SOURCE_DIR}/Modules/InstallRequiredSystemLibraries.cmake)
 endif()
 
-set(CPACK_RESOURCE_FILE_LICENSE "${CMake_LICENSE_FILE}")
+set(CPACK_RESOURCE_FILE_LICENSE "${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/LICENSE.txt")
+configure_file("${CMake_LICENSE_FILE}" "${CPACK_RESOURCE_FILE_LICENSE}" COPYONLY)
+
 set(CPACK_PACKAGE_DESCRIPTION_SUMMARY "CMake is a build tool")
 set(CPACK_PACKAGE_VENDOR "Kitware")
 set(CPACK_PACKAGE_DESCRIPTION_FILE "${CPACK_RESOURCE_FILE_LICENSE}")