Make buffer size configurable at compile time and set it to 8192

[jcorporation]

edit: I notified meanwhile the default mpd output buffer is 8 MiB not 8 kiB

The libmpdclient buffer is the max length of a output line and this is not limited on server side?

But eventually this patch is even so useful?

Or I struggling with all of this?

[MaxKellermann]

I don't think a compile-time option is a good idea. Rarely ever will people do their own libmpdclient build, and adding that compile-time knob will make us believe the problem is fixed, when in practice it's not for most people. We need a fallback to dymamic allocation like #53 (which was not yet merged because the code was too complicated and fragile.)