Require webm when explicitly requested

https://code.google.com/p/webm/issues/detail?id=906

Change-Id: I72841078ff81152d21d84ccf4d5548e757685a6d

vpxenc.c

@@ -183,8 +183,10 @@ static const arg_def_t recontest = ARG_DEF_ENUM(
     NULL, "test-decode", 1, "Test encode/decode mismatch", test_decode_enum);
 static const arg_def_t framerate = ARG_DEF(
     NULL, "fps", 1, "Stream frame rate (rate/scale)");
+static const arg_def_t use_webm = ARG_DEF(
+    NULL, "webm", 0, "Output WebM (default when WebM IO is enabled)");
 static const arg_def_t use_ivf = ARG_DEF(
-    NULL, "ivf", 0, "Output IVF (default is WebM if WebM IO is enabled)");
+    NULL, "ivf", 0, "Output IVF");
 static const arg_def_t out_part = ARG_DEF(
     "P", "output-partitions", 0,
     "Makes encoder output partitions. Requires IVF output!");
@@ -208,7 +210,7 @@ static const arg_def_t *main_args[] = {
   &debugmode,
   &outputfile, &codecarg, &passes, &pass_arg, &fpf_name, &limit, &skip,
   &deadline, &best_dl, &good_dl, &rt_dl,
-  &quietarg, &verbosearg, &psnrarg, &use_ivf, &out_part, &q_hist_n,
+  &quietarg, &verbosearg, &psnrarg, &use_webm, &use_ivf, &out_part, &q_hist_n,
   &rate_hist_n, &disable_warnings, &disable_warning_prompt,
   NULL
 };
@@ -1058,6 +1060,12 @@ static int parse_stream_params(struct VpxEncoderConfig *global,
 #if CONFIG_FP_MB_STATS
     } else if (arg_match(&arg, &fpmbf_name, argi)) {
       config->fpmb_stats_fn = arg.val;
+#endif
+    } else if (arg_match(&arg, &use_webm, argi)) {
+#if CONFIG_WEBM_IO
+      config->write_webm = 1;
+#else
+      die("Error: --webm specified but webm is disabled.");
 #endif
     } else if (arg_match(&arg, &use_ivf, argi)) {
       config->write_webm = 0;