added in linux kernel commit 73fa7547c70b32cc69685f79be31135797734eb6.
this is added now as a prerequisite for fixing pwrite/pwritev behavior
for O_APPEND files.
these badly pollute the namespace with macros whenever _GNU_SOURCE is
defined, which is always the case with g++, and especially tends to
interfere with C++ constructs.
as our implementation of these was macro-only, their removal cannot
affect any existing binaries. at the source level, portable software
should be prepared for them not to exist.
for now, they are left in place with explicit _LARGEFILE64_SOURCE.
this provides an easy temporary path for integrators/distributions to
get packages building again right away if they break while working on
a proper, upstreamable fix. the intent is that this be a very
short-term measure and that the macros be removed entirely in the next
release cycle.
while there's no POSIX namespace provision for UIO_* in uio.h, this
exact macro name is reserved in XBD 2.2.2. apparently some
glibc-centric software expects it to exist, so let's provide it.
the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.
installation docs have also been updated to reflect this change.
