Unix port: add support for the Hurd.

* ports/unix/port/include/arch/cc.h:
        Define LWIP_ERRNO_STDINCLUDE for Linux and Hurd, as defining
        LWIP_ERRNO_INCLUDE doesn't work on these systems.
    * ports/unix/port/sys_arch.c:
        Rename pthread_cond_wait() and pthread_cond_timedwait()
        to use the Hurd's non-portable versions on GNU systems.

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

@@ -43,6 +43,8 @@
 #define LWIP_UNIX_OPENBSD
 #elif defined __CYGWIN__
 #define LWIP_UNIX_CYGWIN
+#elif defined __GNU__
+#define LWIP_UNIX_HURD
 #endif
 
 #define LWIP_TIMEVAL_PRIVATE 0
@@ -50,6 +52,10 @@
 
 #define LWIP_ERRNO_INCLUDE <errno.h>
 
+#if defined(LWIP_UNIX_LINUX) || defined(LWIP_UNIX_HURD)
+#define LWIP_ERRNO_STDINCLUDE	1
+#endif
+
 #define LWIP_RAND() ((u32_t)rand())
 
 /* different handling for unit test, normally not needed */

ports/unix/port/sys_arch.c

@@ -477,6 +477,11 @@ cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex, u32_t timeout)
   struct timespec rtime1, rtime2, ts;
   int ret;
 
+#ifdef __GNU__
+  #define pthread_cond_wait pthread_hurd_cond_wait_np
+  #define pthread_cond_timedwait pthread_hurd_cond_timedwait_np
+#endif
+
   if (timeout == 0) {
     pthread_cond_wait(cond, mutex);
     return 0;