build: set _GNU_SOURCE when checking for SO_PEERCRED
SO_PEERCRED is not POSIX, so might be hidden unless _GNU_SOURCE is defined.
See 1: cc.has_header_symbol() does not inherit the project arguments.
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SO_PEERCRED is not POSIX, so might be hidden unless _GNU_SOURCE is defined.
See 1: cc.has_header_symbol() does not inherit the project arguments.