CMake: Include InstallRequiredSystemLibraries normally

This includes this module normally and enables changing
InstallRequiredSystemLibraries independently of the minimum CMake
version specified in the CMake's root CMakeLists.txt, when building
CMake itself with CMake.

CMakeCPack.cmake

@@ -11,7 +11,7 @@ if(NOT DEFINED CMAKE_INSTALL_SYSTEM_RUNTIME_LIBS_NO_WARNINGS)
 endif()
 
 if(CMake_INSTALL_DEPENDENCIES)
-  include(${CMake_SOURCE_DIR}/Modules/InstallRequiredSystemLibraries.cmake)
+  include(InstallRequiredSystemLibraries)
 endif()
 
 set(CPACK_RESOURCE_FILE_LICENSE "${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/LICENSE.txt")