Added "header guard" to prevent multiple definitions of the library target.

CMakeLists.txt

@@ -2,6 +2,12 @@ cmake_minimum_required(VERSION 3.03)
 
 project(tcpip VERSION 0.1)
 
+# If the target already exists in a project (e.g. if tcpip is included multiple time as a submodule of submodules),
+# this "header guard"-style check prevents defining the same target multiple times (which will lead to CMake errors).
+if(TARGET tcpip)
+  return()
+endif()
+
 add_compile_options(
   -Wall
   -Wformat=2