doc/developer: Clarify symbol naming prefixes section.
configure: Enable hwaccels without external dependencies by default.
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  configure: Enable hwaccels without external dependencies by default.
  doc/developer: Clarify symbol naming prefixes section.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Fix compilation with --disable-mmx.