mdb_compat(3) on Linux
The text 744170e044f7, as Debian 12 (Bookworm), Debian 11 (Bullseye) shipped it.
A bunch of macros to minimize the amount of platform-specific ifdefs needed throughout the rest of the code. When the features this library needs are similar enough to POSIX to be hidden in a one-or-two line replacement, this macro approach is used.