[mythtv] New configure script automatically enabling XVMC

[Anduin Withers]

> Pulled from CVS last night.  Running the configure script without any
> special options automatically added XVMC and XVMC-vld.  Net result was
> that going to Watch TV or Watch Recordings resulted in a black screen.

The xvmc_vld check seems to be enabled by default, for you to hit this you'd
have to have libviaXvMC installed but not want to use it.

Having it default to no seems better (and is closer to the old settings.pro
behavior).

-- 
Anduin Withers