[Geneva] error out for missing dpes when building as external component (open-telemetry#565)

Author: ThomsonTan

exporters/fluentd/CMakeLists.txt

@@ -41,12 +41,16 @@ set(nlohmann_json_clone FALSE)
 if(nlohmann_json_FOUND)
   message("Using external nlohmann::json")
 else()
-  include(cmake/nlohmann-json.cmake)
-  set(nlohmann_json_clone TRUE)
-  set(nlohmann_json_SOURCE_DIR
-      "${CMAKE_SOURCE_DIR}/nlohmann_json/single_include")
-  include_directories(${nlohmann_json_SOURCE_DIR})
-  message("nlohmann_json package was not found. Cloning from github")
+  if (MAIN_PROJECT)
+    include(cmake/nlohmann-json.cmake)
+    set(nlohmann_json_clone TRUE)
+    set(nlohmann_json_SOURCE_DIR
+        "${CMAKE_SOURCE_DIR}/nlohmann_json/single_include")
+    include_directories(${nlohmann_json_SOURCE_DIR})
+    message("nlohmann_json package was not found. Cloning from github")
+  else()
+    message(FATAL_ERROR "nlohmann_json package was not found which is required for opentelemetry-cpp-fluentd. Please install it")
+  endif()
 endif()
 
 if(MAIN_PROJECT)