macosx dialog provider: relicense to LGPLv2.1+ for use within VLCKit

Agreed by Pierre d'Herbemont, Sebastian Zwickert, Rémi Denis-Courmont, Jean-Baptiste Kempf and me
build system: add conditional variables to differenciate between iOS and OS X

This allows automagic enabling and disabling of the needed and useful aout and vout modules
configure: fix compilation on non-Darwin platforms