AOSP: lib/support: don't assume qsort_r() is always available on Linux

AOSP: lib/support: don't assume qsort_r() is always available on Linux

Since commit 4e5f24ae4267 ("Use an autoconf test to detect for a BSD- or
GNU-style qsort_r function"), e2fsck fails to build for Android because
lib/support/sort_r.h assumes that qsort_r() is always available on
"Linux", but in fact it's not supported by Android's libc.

Rename _SORT_R_LINUX to _SORT_R_GNU to clarify that it's really the
glibc convention for qsort_r(), not the "Linux" convention per se, and
make sort_r.h stop setting it automatically when __linux__ is defined.

Note: this change does *not* prevent glibc's qsort_r() from being used
when e2fsprogs is built using the autotools-based build system, as
'configure' checks for qsort_r() too.  This change just affects the
fallback behavior for when qsort_r() was not already detected.

Fixes: 4e5f24ae4267 ("Use an autoconf test to detect for a BSD- or GNU-style qsort_r function")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20230130215829.863455-1-ebiggers@kernel.org
Change-Id: I4ed2fd6aef5a0d62960988d29e35acd337bb7d02
From AOSP commit: 9f289d0add4f12fa2e4b21754141363a2759d152