Fix issue #1852: manuals needs more explicit dependencies.
Dependencies are already being tracker by lilypond-book.py,
which produces .dep files containing make rules. The file
stepmake/stepmake/generic-targets.make includes them into
the build.
The problem is that the .dep files contain errors.
We fix the .dep files produced by lilypond-book.py by
1) using the relative path to the target file,
2) using the correct file extension for the target file.