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
Build: Disallow metapost versions 1.600 <= x < 1.803.

Add a check in ./configure to avoid buggy versions of metapost
starting with the rewrite that occured in the 1.600 series and
finally fixed in version 1.803 and later.

Fixes issue 3539: Configure should forbid mpost 1.802 from TexLive 2013
http://code.google.com/p/lilypond/issues/detail?id=3539
Build: Document the default make target.

Update `make help' to correctly document the generic targets.
Also add `local-all' target and mark the targets as PHONY.