This relies contrib version of x262 that gives x262.h and all. Otherwise
it seems to be pretty hard to have x264/x26410b/x262 in the same time.

Use Write-function and refactored functions to check that in flushing
we don't write too long last segment but split it in multiple segments
if needed. Fixes #9383

extras/tools: disable contrib compilation of gpg-error, gcrypt and lua since we don't use it for now

macosx: Update Sparkle Appcast URL as we won't provide fat binaries for the foreseeable future anymore

This reverts commit ce972e18.

No functionnal change

See http://wiki.xiph.org/Chapter_Extension