BUILD: lua: replace timegm() with my_timegm() to fix build on Solaris 10

Akhnin Nikita reported that Lua doesn't build on Solaris 10 because
the code uses timegm() to parse a date, which is not provided there.
The recommended way to implement timegm() is broken in the man page,
as it is based on a change of the TZ environment variable at run time
before calling the function (which is obviously not thread safe, and
terribly inefficient).

Here instead we rely on the new my_timegm() function, it should be
sufficient for all known use cases.

src/hlua_fcn.c

@@ -287,7 +287,7 @@ static int hlua_parse_date(lua_State *L, int (*fcn)(const char *, int, struct tm
 	 * the timezone from the broken-down time, it must be fixed
 	 * after the conversion.
 	 */
-	time = timegm(&tm);
+	time = my_timegm(&tm);
 	if (time == -1) {
 		lua_pushnil(L);
 		return 1;