CMake: Provide proper interface targets from dependencies

* to make use of cmakes inherited link dependencies which
  imports all include directories/ definitions from the link library

dep/utf8cpp/CMakeLists.txt

@@ -0,0 +1,15 @@
+# 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.
+
+add_library(utf8cpp INTERFACE)
+
+target_include_directories(utf8cpp
+  INTERFACE
+    ${CMAKE_CURRENT_SOURCE_DIR})