extras/contrib/src/Makefile: cleaned up INSTALL_NAME macro for MacOS X: all the dylib in contrib/lib are soft-linked to their distributed version in 'vlc-lib'. this way, VLC build system can use contrib libs on MacOS X the same way as for UNIX, and should simplify porting to libtool
configure.ac: cleaned up Cocoa.h detection, removed link path dependency to 'vlc-lib' as it is no longer necessary
Makefile.am: fixed installation of dylibs as vlc_ prefix is already appended by INSTALL_NAME

NOTE: this change is highly disruptive for MacOS X build, I strongly recommend that you rebuild the contrib packages for this platform. Other platforms are unaffected