Use -pthread where and only where needed

Passing -pthread to GCC as a flag makes it both link to libpthread, and make C standard library routines reentrant. This makes the additional explicit links unnecessary.

Additionally, on OSX, this is the default behavior, and clang will print a message about it being unused if it's present there.

CMakeLists.txt

@@ -6,7 +6,11 @@ project(citra)
 
 if (NOT MSVC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wno-attributes")
-    add_definitions(-pthread)
+    
+    if (NOT APPLE)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -pthread")
+    endif()
 else()
     # Silence deprecation warnings
     add_definitions(/D_CRT_SECURE_NO_WARNINGS)