client: Use poll() as fallback when ppoll() is unavailable

On platforms like darwin that lack ppoll(), fall back to poll() with
a millisecond timeout. Since the ppoll() call uses a NULL signal mask,
the behavior is equivalent.

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>

meson.build

@@ -51,6 +51,7 @@ have_funcs = [
 	'getpeereid',
 	'mkostemp',
 	'posix_fallocate',
+	'ppoll',
 	'prctl',
 	'memfd_create',
 	'mremap',