Unify timegm fallbacks into my_mktime

my_timegm was introduced in 1900c7 to add a mktime fallback used in
4a6241, the subsequently removed in c53823. As a result of this code
shuffling, the jq mktime function (f_mktime) wound up using a fallback
even on systems where timegm is available, with incorrect results in
some time zones with DST, e.g.

% TZ=America/New_York jq -n '"2018-08-31T00:00:00Z"|fromdate|todate'
"2018-08-31T01:00:00Z"

This undoes 1900c7 and moves the workaround it added into the #else of
my_mktime.

src/builtin.c

@@ -1225,10 +1225,21 @@ static int setenv(const char *var, const char *val, int ovr)
  *
  * Returns (time_t)-2 if mktime()'s side-effects cannot be corrected.
  */
-static time_t my_timegm(struct tm *tm) {
+static time_t my_mktime(struct tm *tm) {
 #ifdef HAVE_TIMEGM
   return timegm(tm);
-#else /* HAVE_TIMEGM */
+#elif HAVE_TM_TM_GMT_OFF
+
+  time_t t = mktime(tm);
+  if (t == (time_t)-1)
+    return t;
+  return t + tm->tm_gmtoff;
+#elif HAVE_TM___TM_GMT_OFF
+  time_t t = mktime(tm);
+  if (t == (time_t)-1)
+    return t;
+  return t + tm->__tm_gmtoff;
+#else
   char *tz;
 
   tz = (tz = getenv("TZ")) != NULL ? strdup(tz) : NULL;
@@ -1238,18 +1249,6 @@ static time_t my_timegm(struct tm *tm) {
   if (tz != NULL)
     setenv("TZ", tz, 1);
   return t;
-#endif /* !HAVE_TIMEGM */
-}
-static time_t my_mktime(struct tm *tm) {
-  time_t t = mktime(tm);
-  if (t == (time_t)-1)
-    return t;
-#ifdef HAVE_TM_TM_GMT_OFF
-  return t + tm->tm_gmtoff;
-#elif HAVE_TM___TM_GMT_OFF
-  return t + tm->__tm_gmtoff;
-#else
-  return (time_t)-2; /* Not supported */
 #endif
 }