* How to run lilypond from the development tree? 

If you want to contribute and can build LilyPond yourself, there's no
need to install that development version; you can run it from the
build tree.

put the following script in your PATH, with $LILY_SRC_DIR set to the
source directory.

  #!/bin/sh

  exec $LILY_SRC_DIR/out/bin/lilypond --relocate "$@"