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.h

@@ -84,5 +84,9 @@ const std::string indentProject (
   const std::string& whitespace = "  ",
   char delimiter = '.');
 
+#ifndef HAVE_TIMEGM
+  time_t timegm (struct tm *tm);
+#endif
+
 #endif
 ////////////////////////////////////////////////////////////////////////////////