compile.sh: earlier abort in case of invalid option

If an invalid option is passed to "-a" it make sense for us to abort
as early as possible. These changes can be summerized with;

  - do not rely on compile-libvlc.sh to diagnose invalid usage, and;
  - abort before any work is done if an invalid option is present.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>