URCU's api_*.h

On Tue, Jun 16, 2009 at 07:09:11PM +0200, Jan Blunck wrote:
> I already did most of that for libkcompat. That was actually why I was
> asking if I should port that over.

Nevertheless, I offer the following patch to userspace-rcu to provide
a gcc-library version of the api_*.h files and to clean up the build a
bit.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>