Add configure guards around literal jv_numbers

Allow building jq in a mode that doesn't use decnumber for benchmarking
purposes. decnumber support is enabled by default, and this option is
meant to be removed once we're happy with the performance.

src/jv_parse.c

@@ -501,11 +501,20 @@ static pfunc check_literal(struct jv_parser* p) {
   } else {
     // FIXME: better parser
     p->tokenbuf[p->tokenpos] = 0;
+#ifdef USE_DECNUM
     jv number = jv_number_with_literal(p->tokenbuf);
     if (jv_get_kind(number) == JV_KIND_INVALID) {
       return "Invalid numeric literal";
     }
     TRY(value(p, number));
+#else
+    char *end = 0;
+    double d = jvp_strtod(&p->dtoa, p->tokenbuf, &end);
+    if (end == 0 || *end != 0) {
+      return "Invalid numeric literal";
+    }
+    TRY(value(p, jv_number(d)));
+#endif
   }
   p->tokenpos = 0;
   return 0;