python include path oddity with "make install"

[Graham Percival]

At the moment, doing:
  mkdir build
  cd build
  ../configure --prefix=$HOME/.local/
  make
  make install

results in python files which can't find lilylib.  This is
installed to:
    $(PREFIX)/share/lilypond/$(VERSION)/python/

The relocation is supposed to be handled by:
    python/relocate-preamble.py.in
but it seems to assume that "current" is a valid $(VERSION).
I know that GUB does add a symlink for "current", but that doesn't
appear to happen for "make install".


I can see a few different ways forward:
- figure out why the @lilypond_datadir@ replacement is going to
  /usr/...  instead of $(PREFIX)
- add a "current" symlink
- add some more directories to the system path in
  relocate-preamble.py.in

Unfortunately, I've lost a lot of steam on this and am not likely
to return to it until Feb.  I'd rather not hold back the
pure-python midi2ly change, so it would be awesome if somebody
else could clarify matters and/or fix it.

Cheers,
- Graham