Support CMake none targets

When trying to import rapidjson with for exemple : 
fetchcontent_declare(rapidjson GIT_REPOSITORY https://github.com/Tencent/rapidjson.git)

if your CMake/Clang is "bare metal", exemple given : 

set(CMAKE_SYSTEM_NAME none)
set(CMAKE_SYSTEM_PROCESSOR x86_64)
set(CMAKE_C_COMPILER_TARGET x86_64-elf-none)
set(CMAKE_CXX_COMPILER_TARGET x86_64-elf-none)

CMake fails to process CMakeLists.txt because of the switch on UNIX/CYGWIN/WIN32 for install directory.

Error is: 

CMake Error at cmake-build-debug-clang/_deps/rapidjson-src/CMakeLists.txt:244 (INSTALL):
  INSTALL FILES given no DESTINATION!

CMakeLists.txt

@@ -241,8 +241,10 @@ INSTALL(FILES "${CMAKE_CURRENT_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/${PROJECT_NAM
         DESTINATION ${CMAKECONFIG_INSTALL_DIR} )
 
 # Install files
-INSTALL(FILES
+IF(CMAKE_INSTALL_DIR)
+  INSTALL(FILES
       ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}Config.cmake
       ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake
       DESTINATION "${CMAKE_INSTALL_DIR}"
       COMPONENT dev)
+ENDIF()