Generate ggo dependencies automatically.

Currently we maintain the dependency list of each ggo file manually in
m4/gengetopt/makefile. This is tedious and error-prone, and missing
dependencies result in build failures which are not repeatable and
hard to locate.

This changes the build system to keep track of these dependencies
automatically. We now create one .m4d file for each executable. This
file lists the dependencies of the corresponding ggo file, i.e. those
m4 files which are included in order to make the ggo.

All .m4d files are pulled in from the main Makefile via the -include
directive, in the same way we include the object dependency files
(.d) generated by gcc.