Continue working on CMake build system

Avoid checking for C++ compiler, not needed

ports/unix/unixsim/CMakeLists.txt

@@ -1,5 +1,7 @@
 cmake_minimum_required(VERSION 3.8)
 
+project(unixsim C)
+
 include(../../CMakeCommon.cmake)
 
 add_definitions(-DLWIP_DEBUG)
@@ -11,8 +13,8 @@ include_directories(lwip PUBLIC
     "${CMAKE_CURRENT_SOURCE_DIR}/./"
 )
 
-add_subdirectory(../../../ contrib)
 add_subdirectory(../../../../lwip/src lwip)
+add_subdirectory(../../../ contrib)
 add_subdirectory(../ unixport)
 
 # lwipcontribaddons is specified multiple times in target_link_libraries because of cyclic dependency