Replace all `#define _*_SOURCE` with centralised config#855

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RReverser wants to merge 9 commits intogphoto:masterfrom RReverser:posix-extensions

+214-206

Commits on Oct 30, 2022

Replace all #define _*_SOURCE with centralised config
RReverser
committed

Commits on Oct 31, 2022

Use _DEFAULT_SOURCE instead
RReverser
committed

Commits on Nov 2, 2022

Revert to AC_USE_SYSTEM_EXTENSIONS
RReverser
authored

Commits on Nov 3, 2022

Commits on Nov 7, 2022