autoconf: Use ":" instead of "dnl" as a noop

Summary:
Fixes #424.

This is a backport of libsecp256k1 PR752

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7588