Fix travis compile error

configure.ac

@@ -556,6 +556,8 @@ fi
 # adjustment.
 AC_CHECK_SIZEOF([int *])
 
+AC_CHECK_SIZEOF([time_t])
+
 # Checks for library functions.
 if test "x$cross_compiling" != "xyes"; then
   AC_FUNC_MALLOC

src/timegm.c

@@ -44,10 +44,12 @@ time_t timegm(struct tm *tm) {
   num_leap_year = count_leap_year(tm->tm_year + 1900) - count_leap_year(1970);
   days = (tm->tm_year - 70) * 365 + num_leap_year + tm->tm_yday;
   t = ((int64_t)days * 24 + tm->tm_hour) * 3600 + tm->tm_min * 60 + tm->tm_sec;
-  if (sizeof(time_t) == 4) {
+
+#if SIZEOF_TIME_T == 4
   if (t < INT32_MIN || t > INT32_MAX) {
     return -1;
   }
-  }
+#endif /* SIZEOF_TIME_T == 4 */
+
   return (time_t)t;
 }