Bug #1030

- Added portable implementation of timegm for non GNU/BSD platforms that
  don't have their own implementation
- Removed the use of tm_gmtoff on non GNU/BSD platforms that don't have
  it as part of the tm struct in time.h
- Added CMake tests HAVE_TIMEGM and HAVE_TM_GMTOFF

src/util.cpp

@@ -610,3 +610,23 @@ const std::string indentProject (
 
 ////////////////////////////////////////////////////////////////////////////////
 
+////////////////////////////////////////////////////////////////////////////////
+#ifndef HAVE_TIMEGM
+time_t timegm (struct tm *tm)
+{
+  time_t ret;
+  char *tz;
+  tz = getenv ("TZ");
+  setenv ("TZ", "UTC", 1);
+  tzset ();
+  ret = mktime (tm);
+  if (tz)
+    setenv ("TZ", tz, 1);
+  else
+    unsetenv ("TZ");
+  tzset ();
+  return ret;
+}
+#endif
+////////////////////////////////////////////////////////////////////////////////
+