Successful LilyPond build

These are the changes I needed to be able to build LilyPond with gub.

- if python3 is the default on the system, some of the gub python scripts do not work, so it is necessary to be able to specify PYTHON=python2, and the makefiles must obey this setting.

- if texinfo 5.1 is the default, stricter syntax checking compared to previous versions leads to build errors. Building the info docs for gcc and guile fails, so I've disabled them.

- if dash is the default shell, it does not play well with texinfo, because it does not allow overriding of built-in functions, but texinfo relies on this indirectly. I've patched texinfo.

- the version of ImageMagick required to build LilyPond is no longer downloadable, so a minor update was in order.

- dblatex, when called from gub, fails for some reason. gub shouldn't be using the system dblatex anyway ([issue 2329](http://code.google.com/p/lilypond/issues/detail?id=2329)) so I disabled the dblatex regression tests until gub builds its own dblatex.

- I ran into a couple python errors that I've fixed (issubclass called on a None object, SyntaxError on lambda(x)).