emacs: Compute build dependencies to fix byte compile issues

Previously, we simply byte compiled each Elisp source file
independently.  This is actually the wrong thing to do and can lead to
issues with macros and performance issues with substitutions because
1) when the byte compiler encounters a (require 'x) form, it will load
x.elc in preference to x.el, even if x.el is newer, and as a result
may load old macro and substitution definitions and 2) if we update a
macro or substitution definition in one file, we currently won't
re-compile other files that depend on the file containing the
definition.

This patch addresses these problems by computing make dependency rules
from the (require 'x) forms in the Elisp source files, which we inject
into make's dependency database.