buildsystem/compile-libvlc.sh: allow build from anywhere

And don't change the directory since this script can be sourced.

It is now possible to use this script for

- Build an existing VLC:
vlc $ ../libvlcjni/buildsystem/compile-libvlc.sh -a arm

- Build VLC from libvlcjni
libvlcjni $ ./buildsystem/compile-libvlc.sh -a arm

- Build VLC from vlc-android