Build unix port on MacOS/clang

Add support to use Darwin byte order functions instead of LwIP's macros.
These were being pulled in through system includes used in the Unix port files.
Pull sys/types.h in cc.h to get the Darwin macros and use them throughout

In sio.c a number of non-POSIX types/symbols are used, by default MacOS is compiling
with _POSIX_C_SOURCE which guards these additional items. To enable them, we define
_DARWIN_C_SOURCE before any system includes

In unixif.c string.h was not included yet memset and friends are being used. This
caused Wimplicit-function-declaration warnings with clang

This only fixes issue seen in the source, the Makefiles need additional changes to
build with MacOS

ports/unix/port/include/arch/cc.h

@@ -62,6 +62,14 @@
 typedef __kernel_fd_set fd_set;
 #endif
 
+#if defined(LWIP_UNIX_MACH)
+/* sys/types.h and signal.h bring in Darwin byte order macros. pull the
+   header here and disable LwIP's version so that apps still can get
+   the macros via LwIP headers and use system headers */
+#include <sys/types.h>
+#define LWIP_DONT_PROVIDE_BYTEORDER_FUNCTIONS
+#endif
+
 struct sio_status_s;
 typedef struct sio_status_s sio_status_t;
 #define sio_fd_t sio_status_t*

ports/unix/port/netif/sio.c

@@ -4,6 +4,14 @@
 #define _XOPEN_SOURCE 600
 #define _GNU_SOURCE
 
+/* build with Darwin C extensions not part of POSIX, i.e. FASYNC, SIGIO.
+   we can't use LWIP_UNIX_MACH because extensions need to be turned
+   on before any system headers (which are pulled in through cc.h)
+   are included */
+#if defined(__APPLE__)
+#define _DARWIN_C_SOURCE
+#endif
+
 #include "netif/sio.h"
 #include "netif/fifo.h"
 #include "lwip/debug.h"

ports/unix/port/netif/unixif.c

@@ -35,6 +35,7 @@
 #include <fcntl.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/uio.h>