For the convenience of Linux package maintainers, add to Makefile.gtk

For the convenience of Linux package maintainers, add to Makefile.gtk
and Makefile.doc a command-line parameter 'BINPREFIX' which will be
prepended to all the game binary names. E.g. 'make BINPREFIX=sgt-' and
'make BINPREFIX=sgt- install', and correspondingly 'make -f
Makefile.doc BINPREFIX=sgt-'.

Also included in this commit by mistake, changes to singles.c to add
\n to the end of all its debug() statements. I meant to commit that
separately. Oops.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9606 cda61777-01e9-0310-a592-d414129be87e