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glibc doesn't require _GNU_SOURCE to expose certain MAP_* constants
anymore (MAP_ANONYMOUS being particularly noteworthy among them for
being on track for POSIX Issue 8). See this commit:
https://sourceware.org/git/?p=glibc.git;a=commit;h=954b8f38958de72c4712088735eb175118f17b51

or my glibc bug report which triggered the change at
https://sourceware.org/bugzilla/show_bug.cgi?id=29375

This is because POSIX reserves MAP_ constants to the implementation, so
their unconditional visibility in sys/mman.h is always allowed. This is
also helpful for portable application writers.

Other C libraries like musl already exhibit this behavior.

Because mmap() is in a POSIX-specific header already, using it with any
combination of MAP_ constants doesn't require feature test macros.

Thanks for your consideration