Import lwIP contrib rep

... from http://git.savannah.gnu.org/cgit/lwip/lwip-contrib.git/ into contrib/ subdir, STABLE-2_1_0_RELEASE tag
lwIP contrib is now officially frozen
TODO: Fix build

contrib/ports/unix/lib/CMakeLists.txt

@@ -0,0 +1,34 @@
+cmake_minimum_required(VERSION 3.8)
+
+project(lwip C)
+
+set (BUILD_SHARED_LIBS ON)
+
+if (NOT CMAKE_SYSTEM_NAME STREQUAL Linux AND NOT CMAKE_SYSTEM_NAME STREQUAL GNU)
+    message(FATAL_ERROR "Lwip shared library is only working on Linux or the Hurd")
+endif()
+
+set(LWIP_CONTRIB_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../../..)
+set(LWIP_DIR         ${CMAKE_CURRENT_SOURCE_DIR}/../../../../lwip)
+
+include(${LWIP_CONTRIB_DIR}/ports/CMakeCommon.cmake)
+
+set (LWIP_DEFINITIONS -DLWIP_DEBUG)
+set (LWIP_INCLUDE_DIRS
+    "${LWIP_DIR}/src/include"
+    "${LWIP_CONTRIB_DIR}/"
+    "${LWIP_CONTRIB_DIR}/ports/unix/port/include"
+    "${CMAKE_CURRENT_SOURCE_DIR}/"
+)
+
+include(${LWIP_CONTRIB_DIR}/ports/unix/Filelists.cmake)
+include(${LWIP_DIR}/src/Filelists.cmake)
+
+add_library(lwip ${lwipnoapps_SRCS} ${lwipcontribportunix_SRCS} ${lwipcontribportunixnetifs_SRCS})
+target_compile_options(lwip PRIVATE ${LWIP_COMPILER_FLAGS})
+target_compile_definitions(lwip PRIVATE ${LWIP_DEFINITIONS} ${LWIP_MBEDTLS_DEFINITIONS})
+target_include_directories(lwip PRIVATE ${LWIP_INCLUDE_DIRS} ${LWIP_MBEDTLS_INCLUDE_DIRS})
+target_link_libraries(lwip ${LWIP_SANITIZER_LIBS})
+
+find_library(LIBPTHREAD pthread)
+target_link_libraries(lwip ${LIBPTHREAD})