Add ISO 8601-specific strptime

Implement `strptime` for systems lacking it, but only supporting the
ISO 8601 format string.

src/builtin.c

@@ -1308,7 +1308,25 @@ static void set_tm_yday(struct tm *tm) {
   tm->tm_yday = yday;
 }
 
-#ifdef HAVE_STRPTIME
+#ifndef HAVE_STRPTIME
+static char *strptime(const char *s, const char *format, struct tm *tm) {
+  if (strcmp(format, "%Y-%m-%dT%H:%M:%SZ"))
+    return NULL;
+
+  int count, end;
+  count = sscanf(s, "%d-%d-%dT%d:%d:%d%n",
+                    &tm->tm_year, &tm->tm_mon, &tm->tm_mday,
+                    &tm->tm_hour, &tm->tm_min, &tm->tm_sec,
+                    &end );
+  if (count == 6 && s[end] == 'Z') {
+    tm->tm_year -= 1900;
+    tm->tm_mon--;
+    return (char*)s + end + 1;
+  }
+  return NULL;
+}
+#endif
+
 static jv f_strptime(jq_state *jq, jv a, jv b) {
   if (jv_get_kind(a) != JV_KIND_STRING || jv_get_kind(b) != JV_KIND_STRING) {
     return ret_error2(a, b, jv_string("strptime/1 requires string inputs and arguments"));
@@ -1320,8 +1338,13 @@ static jv f_strptime(jq_state *jq, jv a, jv b) {
   tm.tm_yday = 367; // sentinel
   const char *input = jv_string_value(a);
   const char *fmt = jv_string_value(b);
-  const char *end = strptime(input, fmt, &tm);
 
+#ifndef HAVE_STRPTIME
+  if (strcmp(fmt, "%Y-%m-%dT%H:%M:%SZ")) {
+    return ret_error2(a, b, jv_string("strptime/1 only supports ISO 8601 on this platform"));
+  }
+#endif
+  const char *end = strptime(input, fmt, &tm);
   if (end == NULL || (*end != '\0' && !isspace(*end))) {
     return ret_error2(a, b, jv_string_fmt("date \"%s\" does not match format \"%s\"", input, fmt));
   }
@@ -1358,13 +1381,6 @@ static jv f_strptime(jq_state *jq, jv a, jv b) {
   jv_free(a); // must come after `*end` because `end` is a pointer into `a`'s string
   return r;
 }
-#else
-static jv f_strptime(jq_state *jq, jv a, jv b) {
-  jv_free(a);
-  jv_free(b);
-  return jv_invalid_with_msg(jv_string("strptime/1 not implemented on this platform"));
-}
-#endif
 
 #define TO_TM_FIELD(t, j, i)                    \
     do {                                        \