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Fix undefined macro usage in cppformat.

Description

Fix undefined macro usage in cppformat.

This is quite noisy with -Wexpansion-to-defined in both clang 3.9 and gcc 7.
(Refs #3190 and #4148. Upstream might have fixed this, but somewhat clean build logs seem worth it.)

Reviewed By: echotangoecho
Differential Revision: https://code.wildfiregames.com/D680