fixup! Support loading so-long.el on top of an earlier version

`defconst' evaluates and sets the value even one exists already.
1 job for scratch/so-long-updates in 55 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
failed #4174


Name Stage Failure
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1