Build: Dependencies for metafont files (issue 779).

Build: Dependencies for metafont files (issue 779).

Write .dep files containing make dependency rules for .mf files,
allowing to simply type `make' to process the fonts after changing
an input'ed file and have the fonts updated with the minimum amount
of processing.

These .dep files are generated by recursively scanning for lines
of the form "input X;" in the .mf files, and looking up these
input'ed files within the mf source directory. The .dep files are
included into the build by stepmake/generic-targets.make.

Fixes issue 779: fonts should be rebuilt upon modifications to mf/
http://code.google.com/p/lilypond/issues/detail?id=779