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

cmake.h.in

@@ -57,6 +57,12 @@ Override PACKAGE_LANGUAGE, then
 /* Found srandom */
 #cmakedefine HAVE_SRANDOM
 
+/* Found tm_gmtoff */
+#cmakedefine HAVE_TM_GMTOFF
+
+/* Found timegm */
+#cmakedefine HAVE_TIMEGM
+
 /* Found the uuid library */
 #cmakedefine HAVE_UUID
 #cmakedefine HAVE_UUID_UNPARSE_LOWER