Cherry-pick 6a73776004 with minor adaptions in d/control and d/rules

Cherry-pick 6a73776004 with minor adaptions in d/control and d/rules

Bundle our own private copy of guile-1.8

Superceded by guile-2.0, the old guile-1.8 was finally removed from Debian
in May 2016.

Unfortunately, LilyPond 2.18.2 still requires Guile 1.8,
and LilyPond 2.19.x and Guile 2 are just not yet ready for each other.
So, it would seem that bundling our own private guile-1.8 is the only
viable option to keep LilyPond 2.18.2 in Debian for the time being.

Hope this would allow LilyPond to re-enter Debian 9 "stretch"!