Embed newlines in AC_MSG_* output

What is the right way to embed newlines in AC_MSG_FAIL or other AC_MSG_*
macros?

I have a wordy message I want to produce when bailing out of configure when
a security feature was requested but can't be provided due to missing
dependencies.  This is for NTP, and previously the option would silently
fall back to building an insecure configuration, but I want to bring it up
to current best practices and croak loudly if a requested security-related
option can't be fulfilled.

I tried:

AC_MSG_FAILURE(
[--enable-openssl-random was used but no suitable SSL library was\
found.  Remove --enable-openssl-random if you wish to build without\
a cryptographically secure RNG.\
WARNING: Use of ntp-keygen without a secure RNG may generate keys\
that are predictable.])

but it still came out as one long line broken randomly depending on
terminal width, where I want to break on words before ~72 chars.

-- 
Cheers,
Dave Hart