Use monotonic clock for clock_t

This makes clock_t insensitive to the system clock being reset by NTP or
the sysadmin, which could previously cause long hangs for instance in
zmq_poll.

configure.in

@@ -254,7 +254,7 @@ LIBZMQ_CHECK_DOC_BUILD
 # Checks for header files.
 AC_HEADER_STDC
 AC_CHECK_HEADERS(errno.h arpa/inet.h netinet/tcp.h netinet/in.h stddef.h \
-stdlib.h string.h sys/socket.h sys/time.h unistd.h limits.h)
+stdlib.h string.h sys/socket.h sys/time.h time.h unistd.h limits.h)
 
 # Check if we have ifaddrs.h header file.
 AC_CHECK_HEADERS(ifaddrs.h, [AC_DEFINE(ZMQ_HAVE_IFADDRS, 1, [Have ifaddrs.h header.])])
@@ -376,7 +376,7 @@ AM_CONDITIONAL(ON_MINGW, test "x$libzmq_on_mingw32" = "xyes")
 
 # Checks for library functions.
 AC_TYPE_SIGNAL
-AC_CHECK_FUNCS(perror gettimeofday memset socket getifaddrs freeifaddrs)
+AC_CHECK_FUNCS(perror gettimeofday clock_gettime memset socket getifaddrs freeifaddrs)
 AC_CHECK_HEADERS([alloca.h])
 LIBZMQ_CHECK_SOCK_CLOEXEC([AC_DEFINE(
                               [HAVE_SOCK_CLOEXEC],

src/clock.cpp

@@ -34,6 +34,10 @@
 #include <sys/time.h>
 #endif
 
+#if defined HAVE_CLOCK_GETTIME
+#include <time.h>
+#endif
+
 zmq::clock_t::clock_t () :
     last_tsc (rdtsc ()),
     last_time (now_us () / 1000)
@@ -61,6 +65,14 @@ uint64_t zmq::clock_t::now_us ()
     double ticks_div = (double) (ticksPerSecond.QuadPart / 1000000);     
     return (uint64_t) (tick.QuadPart / ticks_div);
 
+#elif defined HAVE_CLOCK_GETTIME
+
+    //  Use POSIX clock_gettime function to get precise monotonic time.
+    struct timespec tv;
+    int rc = clock_gettime (CLOCK_MONOTONIC, &tv);
+    errno_assert (rc == 0);
+    return (tv.tv_sec * (uint64_t) 1000000 + tv.tv_nsec / 1000);
+
 #else
 
     //  Use POSIX gettimeofday function to get precise time.