Fix GnuTLS compilation by backporting our fixes from trunk including the addition of a native opencdk

There is no difference to svn's HEAD. Additionally, svn access is currently blocked.

For Mac OS X 10.4, we need to compile our own GNU Make now.

Needs heavy testing on Win32 and PowerPC-based Macs

Needs testing on Win32 with Cygwin

Code cleanup without semantics changes
Resets the reference count when returning a previously allocated picture
Check error path

--
Rafaël Carré <rcarre@m2x.nl>

commit dc9057ec9b4bb41a74bb07197c64cee8c1976fe0
Author: Rafaël Carré <funman@videolan.org>
Date:   Mon May 19 10:08:04 2008 +0200

    Do not install swscale headers if it was not enabled
Signed-off-by: Jean-Paul Saman <jpsaman@videolan.org>

Cherry-picked (with manual merge fix) from a6c8ae35.

Signed-off-by: Jean-Paul Saman <jpsaman@videolan.org>