macosx/framework: Use local variables where needed. That prevent wrong linkage with an other libvlc, and fix the multiple crash resulting in linking to 2 differents libvlc.

This is a theoretical bug fix, as POLLPRI is not used anywhere in VLC.

contribs: really fix ffmpeg compilation on Darwin (by reverting most of [25028] and adapting its 2nd hunk to fit the upstream code)

fix zsh completion generator, add a script to help compilation, generation, and installation of completion