CMake: Provide proper interface targets from dependencies

dep/threads/CMakeLists.txt

@@ -0,0 +1,16 @@
+# Copyright (C) 2008-2016 TrinityCore <http://www.trinitycore.org/>
+#
+# This file is free software; as a special exception the author gives
+# unlimited permission to copy and/or distribute it, with or without
+# modifications, as long as this notice is preserved.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY, to the extent permitted by law; without even the
+# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+
+find_package(Threads REQUIRED)
+
+add_library(threads INTERFACE)
+target_link_libraries(threads
+  INTERFACE
+    ${CMAKE_THREAD_LIBS_INIT})