Don't test that vlc --help and vlc --version works because automake thinks that any output on stderr is an error.

Remove code duplication

Define DATA_PATH in the root CMakeList.txt since modules might need it (for example the http intf or the lua module)

reported by Felipe Manzano and Anibal Sacoo from Core Security Technologies.

contrib: Setting DYLD_LIBRARY_PATH is bad because it results most of the time in library mismatch. This is an old hack from the past which shouldn't be needed anymore.

cmake: Use CPack to create the package. (Use `make package` or `cpack`). Should produce a tgz/tar.gz/rpm/dmg/nsis. This probably need some tuning.

cmake: Enable testing. (Launch with `make test` or with `ctest` (see ctest --help)). Plus minor cleanup.

FindFFmpeg.cmake: Perform the ffmpeg search only once. (That's probably not the cmake's spirit... to check).