configure: don't use autogen.sh if it doesn't exist

Tarballs don't ship autogen.sh file but configure, so fallback
to the latter if the former doesn't exist when warning the user.
Merge pull request #310 from knocte/tarball

configure: don't use autogen.sh if it doesn't exist
Unicode letters & operators

examples:
let ★ x = x + 5
let (|★) x y = x + y
let summ = ★ 5 |★ 5
comment about \So infix operators