OpenBSD: Fix system feature definitions

Rafael Sadowskirequested to merge
sizeofvoid/cmake:fix/openbsd-support into master

Since !4477 (merged), some sources explicitly enable needed system APIs on some platforms using definitions like _POSIX_C_SOURCE and _XOPEN_SOURCE. Drop the definitions for OpenBSD, which provides the POSIX APIs by default.

Update the definitions for OpenBSD using patches from the official OpenBSD package (called ports).

Backport: release
Topic-rename: openbsd-defines

Edited by Brad King

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