[CMake] Not require librt for building in Mac OS X

Ref #288.

CMakeLists.txt

@@ -16,6 +16,21 @@
 project(husky)
 cmake_minimum_required(VERSION 3.0.2)
 
+# Check building system
+if(WIN32)
+  set(WINDOWS TRUE)
+elseif(UNIX AND NOT APPLE)
+  if(CMAKE_SYSTEM_NAME MATCHES ".*Linux")
+    set(LINUX TRUE)
+  endif()
+elseif(APPLE)
+  if(CMAKE_SYSTEM_NAME MATCHES ".*Darwin.*")
+    set(DARWIN TRUE)
+  elseif(CMAKE_SYSTEM_NAME MATCHES ".*MacOS.*")
+    set(MACOSX TRUE)
+  endif()
+endif()
+
 # CMake Options
 set(CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_STANDARD 14)

cmake/dep.cmake

@@ -96,12 +96,13 @@ else(MONGOCLIENT_FOUND)
     message (STATUS "Could NOT find MongoClient")
 endif(MONGOCLIENT_FOUND)
 if(WITHOUT_MONGODB)
-    unset(MONGOCLIENT_FOUND)
+    set(MONGOCLIENT_FOUND false)
     message(STATUS "Not using MongoClient due to WITHOUT_MONGODB option")
 endif(WITHOUT_MONGODB)
 
 ### RT ###
 
+if(MONGOCLIENT_FOUND AND NOT MACOSX)  # Mac OS X does not have librt
 find_library(RT_LIBRARY NAMES rt)
 if(RT_LIBRARY)
     set(RT_FOUND true)
@@ -114,6 +115,7 @@ if (RT_FOUND)
 else(RT_FOUND)
     message (STATUS "Could NOT find RT")
 endif(RT_FOUND)
+endif(MONGOCLIENT_FOUND AND NOT MACOSX)
 
 ### Thrift ###