Skipping installation of .el.gz files

[Spencer Baugh]

At my site, I'd like to install the Emacs source code along with Emacs
and set source-directory, so that users can easily jump to function
definitions and search the Emacs source.

This works great for C function definitions, but Lisp function
definitions still jump to the .el.gz files which are installed by "make
install".  If I delete those files, jumping to Lisp function definitions
stops working entirely.

Is there a clean and supported way to teach Emacs to jump to the files
in source-directory instead of the .el.gz ones?  This is what it
normally does when I run Emacs without installing it, so I expect it's
possible.