Change Snippets compilation

Change Snippets compilation

- clean up input/lsr/GNUmakefile

- compile Snippets (input/lsr) in Info, rename 'snippets' to
'lilypond-snippets' to avoid possible name clash in /usr/share/info,

- move local-install-info target from Documentation/user/GNUmakefile
to texinfo-targets.make,

- lilypond-book: fix output_print_filename (use format argument
iso. global options format) and call this function properly for
Texinfo snippet formatting,

- add generic purpose @rlsr macro; immediate use is for linking to
snippets tags, but it could be used later to link to individual
snippets,

- www_post.py, add_html_footer.py: add ad-hoc code to make xrefs
to Snippets work.