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/Filelists.cmake

@@ -0,0 +1,40 @@
+# This file is indended to be included in end-user CMakeLists.txt
+# include(/path/to/Filelists.cmake)
+# It assumes the variable LWIP_CONTRIB_DIR is defined pointing to the
+# root path of lwIP contrib sources.
+#
+# This file is NOT designed (on purpose) to be used as cmake
+# subdir via add_subdirectory()
+# The intention is to provide greater flexibility to users to 
+# create their own targets using the *_SRCS variables.
+
+set(lwipcontribportunix_SRCS
+    ${LWIP_CONTRIB_DIR}/ports/unix/port/sys_arch.c
+    ${LWIP_CONTRIB_DIR}/ports/unix/port/perf.c
+)
+
+set(lwipcontribportunixnetifs_SRCS
+    ${LWIP_CONTRIB_DIR}/ports/unix/port/netif/tapif.c
+    ${LWIP_CONTRIB_DIR}/ports/unix/port/netif/list.c
+    ${LWIP_CONTRIB_DIR}/ports/unix/port/netif/sio.c
+    ${LWIP_CONTRIB_DIR}/ports/unix/port/netif/fifo.c
+)
+
+add_library(lwipcontribportunix EXCLUDE_FROM_ALL ${lwipcontribportunix_SRCS} ${lwipcontribportunixnetifs_SRCS})
+target_include_directories(lwipcontribportunix PRIVATE ${LWIP_INCLUDE_DIRS} ${LWIP_MBEDTLS_INCLUDE_DIRS})
+target_compile_options(lwipcontribportunix PRIVATE ${LWIP_COMPILER_FLAGS})
+target_compile_definitions(lwipcontribportunix PRIVATE ${LWIP_DEFINITIONS} ${LWIP_MBEDTLS_DEFINITIONS})
+target_link_libraries(lwipcontribportunix PUBLIC ${LWIP_MBEDTLS_LINK_LIBRARIES})
+
+if (CMAKE_SYSTEM_NAME STREQUAL Linux)
+    find_library(LIBUTIL util)
+    find_library(LIBPTHREAD pthread)
+    find_library(LIBRT rt)
+    target_link_libraries(lwipcontribportunix PUBLIC ${LIBUTIL} ${LIBPTHREAD} ${LIBRT})
+endif()
+
+if (CMAKE_SYSTEM_NAME STREQUAL Darwin)
+    # Darwin doesn't have pthreads or POSIX real-time extensions libs
+    find_library(LIBUTIL util)
+    target_link_libraries(lwipcontribportunix PUBLIC ${LIBUTIL})
+endif()