m4_ifnblank requires autoconf 2.64, and doesn’t seem necessary here.

Submitted upstream to autoconf-archive (no response yet):
https://savannah.gnu.org/patch/?7741



Signed-off-by: Anders Kaseorg <andersk@mit.edu>

This reverts commit df54a869.

This takes too long and the UI is not pretty (since lasts more than 1 s).

These default respectively to yes, yes, no.

Fixes #58 github issue.