sparse: Configure target operating system and fix fallout.

sparse: Configure target operating system and fix fallout.

cgcc, the "sparse" wrapper that OVS uses, can be told the host architecture
or the host OS or both.  Until now, OVS has told it the host architecture
because it is fairly common that it doesn't guess it automatically.  Until
now, OS has not told it the host OS, assuming that it would get it right.
However, it doesn't--if you tell it the host OS or the host architecture,
it doesn't really have a default for the other.  This means that on Linux
(presumably the only OS where sparse works properly for OVS), it was not
defining __linux__, which caused some weird behavior.

This commit adds a flag to the cgcc invocation to make it define __linux__
on Linux, and it fixes some errors that this would otherwise cause.

Acked-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>