This is nicer for cross-compiling, because AC_RUN_IFELSE can't work
there. In practice abstract sockets are supported on Linux since
2.2 (so, all relevant versions), and on no other platform; so it
seems futile to keep this complexity.
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=34905
Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>