Detect Autoconf run in preprocessor?

[Jeffrey Walton]

We are trying to find a solution for
http://lists.gnu.org/archive/html/autoconf/2016-07/msg00003.html.

We are contemplating detecting an Autoconf run in the preprocessor,
and then (if its Autoconf) disabling the test that triggers the
'#error`.

Is there a preprocessor macro defined by Autotools to detect when
software is being configured by the tool? My apologies for asking; I
cannot simply run `g++ -dM -E - </dev/null` because it will lack
Autoconf specific defines.

Thanks in advance.