Fix issue #1852: manuals needs more explicit dependencies.

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.