Remove fallback for _XEatDataWords, require libX11 1.6 for it

_XEatDataWords was orignally introduced with the May 2013 security
patches, and in order to ease the process of delivering those,
fallback versions of _XEatDataWords were included in the X extension
library patches so they could be applied to older versions that didn't
have libX11 1.6 yet.   Now that we're past that hurdle, we can drop
the fallbacks and just require libX11 1.6 for building new versions
of the extension libraries.

Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>