chatgui: add CMake build for libutun (C library linked into GUI)

- Add tools/chatgui/libutun/CMakeLists.txt — builds libuasync (from lib/*.c)
  and libutun (from src/*.c except utun.c) as static libraries
- All C code compiled as C99, linked into chatgui C++ binary
- Include paths and defs match autotools build
- chatgui CMakeLists.txt: add_subdirectory(libutun), link utun library
- Fix zxing-cpp docs/ directory issue

tools/chatgui/zxing-cpp/docs/CMakeLists.txt

@@ -0,0 +1 @@
+