Do not require autotools

Print a warning and try to continue if autotools is not installed.

Fixes IndexError: list index out of range in latestAutomake = automake[-1][1] in shells.py when building libffi if autotools is not installed. An alternative solution would be to add autotools as a dependency of libffi, but it isn't actually needed to build if we are okay not running autoreconf (and this might not be the only package that triggers this error anyway).