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

Summary:
Fixes #424.

This is a backport of libsecp256k1 [[https://github.com/bitcoin-core/secp256k1/pull/752 | PR752]]

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

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

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