message: cmake: add option to use system rapidjson if exists
diff: @@ -7,6 +7,7 @@ include(DefaultCMakeBuildType)
  # Required Clang version
  option(LLVM_ENABLE_RTTI "-fno-rtti if OFF. This should match LLVM libraries" OFF)
  option(USE_SHARED_LLVM "Link against libLLVM.so instead separate LLVM{Option,Support,...}" OFF)
  +option(USE_SYSTEM_RAPIDJSON "Use system RapidJSON instead of the git submodule if exists" ON)
  # Sources for the executable are specified at end of CMakeLists.txt
  add_executable(ccls "")
  @@ -95,9 +96,15 @@ target_compile_definitions(ccls PRIVATE
  ### Includes
  target_include_directories(ccls PRIVATE src)
  -target_include_directories(ccls SYSTEM PRIVATE
  - third_party
  - third_party/rapidjson/include)
  +target_include_directories(ccls SYSTEM PRIVATE third_party)
  +
  +if(USE_SYSTEM_RAPIDJSON)
  + find_package(RapidJSON QUIET)
  +endif()
  +if(NOT RapidJSON_FOUND)
  + set(RapidJSON_INCLUDE_DIRS third_party/rapidjson/include)
  +endif()
  +target_include_directories(ccls SYSTEM PRIVATE ${RapidJSON_INCLUDE_DIRS})
  ### Install
  