emacs: Avoid rebuilding .eldeps even when there's nothing to do

Previously, we updated .eldeps only if the file contents actually
needed to change.  This was done to avoid unnecessary make restarts
(if the .eldeps rule changes the mtime of .eldeps, make has to restart
to collect the new dependencies).  However, this meant that, after a
modification to any .el file that did not change dependencies, .eldeps
would always be out of date, so every make invocation would run the
.eldeps rule, which is both expensive because it starts up Emacs and
noisy.  This was true even when there was nothing to do.  E.g.,

$ make clean && make
...
$ touch emacs/notmuch-lib.el && make
...
$ make
Use "make V=1" to see the verbose compile lines.
EMACS emacs/.eldeps
make: Nothing to be done for `all'.
$ make
Use "make V=1" to see the verbose compile lines.
EMACS emacs/.eldeps
make: Nothing to be done for `all'.

Fix this by replacing .eldeps with two files with identical content.
One tracks the mtime of the dependency information and triggers the
Emacs call to rebuild dependencies only when it may be necessary.  The
other tracks the content only; this rule over-triggers in the same way
the old rule did, but this rule is cheap and quiet.

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