"cmake -E env" was added in CMake 3.1, but we currently support 2.8.12
at minimum. Add a best-effort replacement for older versions. There are
some limitations from CMake (see comments), but these should not affect
the current user (FindAsciidoctor.cmake).
Fixes: v2.5.1rc0-76-g94a0f7c641 ("Switch from AsciiDoc to Asciidoctor.")
Petri-Dish: Peter Wu <firstname.lastname@example.org>
Tested-by: Petri Dish Buildbot
Reviewed-by: Gerald Combs <email@example.com>