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This code piece is adapted from https://github.com/KDE/krita/blob/451e1415fd1b3a0f9c24de2fd4707d1b1e164236/cmake/modules/CheckAtomic.cmake#L23

Fixes: f502ccbc
Fixes: https://bugs.gentoo.org/828065

Tested-by: Yixun Lan <dlan@gentoo.org>
Reviewed-by: Daniel Black