elm engine - restore explicit use of ELM_ENGNE env var

elm engine - restore explicit use of ELM_ENGNE env var

this fixes a deprecation of elm engine done just before 1.13 release
that ends up breaking "make doc" and also will make ELM_ENGINE env var
stop working (the latter was intentional). but since this is relied on
for things like make doc - keep the env var and haveit set preferred
engine and let people migrate away from using the engine var EXCEPT
for things like the shot engine

@fix