BUILD: debug/cli: condition test of O_ASYNC to its existence

BUILD: debug/cli: condition test of O_ASYNC to its existence

David Carlier reported a build breakage on Haiku since commit
5be7c198e ("DEBUG: cli: add a new "debug dev fd" expert command")
due to O_ASYNC not being defined. Ilya also reported it broke the
build on Cygwin. It's not that portable and sometimes defined as
O_NONBLOCK for portability. But here we don't even need that, as
we already condition other flags, let's just ignore it if it does
not exist.

(cherry picked from commit 410942b92a2687f690c886ae1fdc7ad69a3640e0)
Signed-off-by: Willy Tarreau <w@1wt.eu>