Repository: youtube-dl
Issue: youtube-dl complains about outdated avconv on Debian
Labels: bug

### Make sure you are using the _latest_ version: run `youtube-dl --version` and ensure your version is _2016.09.15_. If it's not read [this FAQ entry](https://github.com/rg3/youtube-dl/blob/master/README.md#how-do-i-update-youtube-dl) and update. Issues with outdated version will be rejected.
- [x] I've **verified** and **I assure** that I'm running youtube-dl **2016.09.15**
### Before submitting an _issue_ make sure you have:
- [x] At least skimmed through [README](https://github.com/rg3/youtube-dl/blob/master/README.md) and **most notably** [FAQ](https://github.com/rg3/youtube-dl#faq) and [BUGS](https://github.com/rg3/youtube-dl#bugs) sections
- [x] [Searched](https://github.com/rg3/youtube-dl/search?type=Issues) the bugtracker for similar issues including closed ones
### What is the purpose of your _issue_?
- [ ] Bug report (encountered problems with youtube-dl)
- [ ] Site support request (request for adding support for a new site)
- [x] Feature request (request for a new functionality)
- [ ] Question
- [ ] Other

---
### The following sections concretize particular purposed issues, you can erase any section (the contents between triple ---) not applicable to your _issue_

---
### If the purpose of this _issue_ is a _bug report_, _site support request_ or you are not completely sure provide the full verbose output as follows:

Add `-v` flag to **your command line** you run youtube-dl with, copy the **whole** output and insert it here. It should look similar to one below (replace it with **your** log inserted between triple ```):

```
$ ./youtube-dl -v 'http://www.cc.com/full-episodes/oqhari/the-daily-show-with-trevor-noah-september-14--2016---hannah-hart-season-21-ep-21153'
[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'-v', u'http://www.cc.com/full-episodes/oqhari/the-daily-show-with-trevor-noah-september-14--2016---hannah-hart-season-21-ep-21153']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2016.09.15
[debug] Python version 2.7.9 - Linux-4.5.0-0.bpo.2-amd64-x86_64-with-debian-8.5
[debug] exe versions: avconv 2.6.9, avprobe 2.6.9, ffmpeg 2.6.9, ffprobe 2.6.9, rtmpdump 2.4
[debug] Proxy map: {}
[ComedyCentral] the-daily-show-with-trevor-noah-september-14--2016---hannah-hart-season-21-ep-21153: Downloading webpage
[ComedyCentral] 6a95eb1a-bf13-453a-97c2-dbb01e774f4a: Downloading info
[ComedyCentral] 98bf6757-1d25-4585-94de-0a4b42dbc077: Extracting information
[ComedyCentral] 98bf6757-1d25-4585-94de-0a4b42dbc077: Downloading video urls
[ComedyCentral] be5b7144-a6e2-4efc-9265-d1f91a7adc95: Extracting information
[ComedyCentral] be5b7144-a6e2-4efc-9265-d1f91a7adc95: Downloading video urls
[ComedyCentral] 66615bd2-93f8-4a9e-ab19-fadc642257a0: Extracting information
[ComedyCentral] 66615bd2-93f8-4a9e-ab19-fadc642257a0: Downloading video urls
[ComedyCentral] a6e78047-c0cb-4162-9a13-455af372d1cb: Extracting information
[ComedyCentral] a6e78047-c0cb-4162-9a13-455af372d1cb: Downloading video urls
[download] Downloading playlist: September 14, 2016 - Hannah Hart
[ComedyCentral] playlist September 14, 2016 - Hannah Hart: Collected 4 video ids (downloading 4 of them)
[download] Downloading video 1 of 4
WARNING: Your copy of avconv is outdated and unable to properly mux separate video and audio files, youtube-dl will download single file media. Update avconv to version 10-0 or newer to fix this.
[debug] Invoking downloader on u'http://viacommtvstrmfs.fplive.net/gsp.comedystor/com/dailyshow/TDS/Season_21/21153/ds_21_153_act1_2a0gi8r18t_1920x1080_5128.mp4'
[download] The Daily Show with Trevor Noah_September 14, 2016_21_21153_September 14, 2016 - Hannah Hart_Act 1-98bf6757-1d25-4585-94de-0a4b42dbc077.mp4 has already been downloaded
[download] 100% of 234.77MiB
[download] Downloading video 2 of 4
WARNING: Your copy of avconv is outdated and unable to properly mux separate video and audio files, youtube-dl will download single file media. Update avconv to version 10-0 or newer to fix this.
[debug] Invoking downloader on u'http://viacommtvstrmfs.fplive.net/gsp.comedystor/com/dailyshow/TDS/Season_21/21153/ds_21_153_act2_ltrb8juyp2_1920x1080_5128.mp4'
[download] The Daily Show with Trevor Noah_September 14, 2016_21_21153_September 14, 2016 - Hannah Hart_Act 2-be5b7144-a6e2-4efc-9265-d1f91a7adc95.mp4 has already been downloaded
[download] 100% of 167.62MiB
[download] Downloading video 3 of 4
WARNING: Your copy of avconv is outdated and unable to properly mux separate video and audio files, youtube-dl will download single file media. Update avconv to version 10-0 or newer to fix this.
[debug] Invoking downloader on u'http://viacommtvstrmfs.fplive.net/gsp.comedystor/com/dailyshow/TDS/Season_21/21153/ds_21_153_act3_d20heeqqlf_1920x1080_5128.mp4'
[download] The Daily Show with Trevor Noah_September 14, 2016_21_21153_September 14, 2016 - Hannah Hart_Act 3-66615bd2-93f8-4a9e-ab19-fadc642257a0.mp4 has already been downloaded
[download] 100% of 174.49MiB
[download] Downloading video 4 of 4
WARNING: Your copy of avconv is outdated and unable to properly mux separate video and audio files, youtube-dl will download single file media. Update avconv to version 10-0 or newer to fix this.
[debug] Invoking downloader on u'http://viacommtvstrmfs.fplive.net/gsp.comedystor/com/dailyshow/TDS/Season_21/21153/ds_21_153_act4_10jl2re3zw_1920x1080_5128.mp4'
[download] The Daily Show with Trevor Noah_September 14, 2016_21_21153_September 14, 2016 - Hannah Hart_Act 4-a6e78047-c0cb-4162-9a13-455af372d1cb.mp4 has already been downloaded
[download] 100% of 94.59MiB
[download] Finished downloading playlist: September 14, 2016 - Hannah Hart
```

---
### Description of your _issue_, suggested solution and other information

On Debian, avconv and friends get symlinked to ffmpeg when you have ffmpeg installed, instead of libav.

youtube-dl therefore tries to use avconv preferentially by default, but notices the version string is not what it would prefer, yielding things like the above log, where it concludes the version of avconv is stale, and changes its behavior.

I can work around this by defining --prefer-ffmpeg in ~/.config/youtube-dl.conf, but it would be nice if youtube-dl could, in addition to parsing the version string out of the first line of the {av,ff}\* utilities, parse the utility name out of the first line, and decide that it's not executing the utility it thinks it is (e.g. determining that libav is not actually available on the system when avconv/avplay/etc report ffmpeg/ffplay/..., versus reporting the same version string for both).