cmake: Fail loudly when found rapidjson is too old (MaskRay#646)

When the rapidjson found by cmake is an older version it defines a
variable RAPIDJSON_INCLUDE_DIRS instead of RapidJSON_INCLUDE_DIRS (MaskRay#455).
According to MaskRay#383 we do not want to make these older version work with
ccls. However currently if the rapidjson found by cmake is an older
version that defined RAPIDJSON_INCLUDE_DIRS, then the cmake invocation
still succeeds but any build command will fail because
RapidJSON_INCLUDE_DIRS was never set properly. This makes the cmake
invocation fail with a relevant error message in this event.

CMakeLists.txt

@@ -151,6 +151,9 @@ set_property(SOURCE src/utils.cc APPEND PROPERTY COMPILE_DEFINITIONS
 
 if(USE_SYSTEM_RAPIDJSON)
   find_package(RapidJSON QUIET)
+  if(NOT DEFINED RapidJSON_INCLUDE_DIRS AND DEFINED RAPIDJSON_INCLUDE_DIRS)
+    message(FATAL_ERROR "RapidJSON version is likely too old. See https://github.com/MaskRay/ccls/issues/455")
+  endif()
 endif()
 if(NOT RapidJSON_FOUND)
   set(RapidJSON_INCLUDE_DIRS third_party/rapidjson/include)