Makefile change for developers

[Yoni Rabkin]

I've introduced a small change to the Makefile to help developers test
their code across Emacs releases.

The Makefile now has a new target named "test_releases". If you call it
while setting the RELEASE_BIN and PREV_RELEASE_BIN variables, it will
compile against both of those versions (but clean after itself) and you
will be able to see if there are any errors. A simple command will do,
such as:

$ make test_releases \
       
RELEASE_BIN=$HOME"/devel/emms-hacking/emacs-releases/emacs-28.1/src/emacs" \
       
PREV_RELEASE_BIN=$HOME"/devel/emms-hacking/emacs-releases/emacs-27.2/src/emacs"

-- 
   "Cut your own wood and it will warm you twice"