Userspace RCU

For automated documentation purposes, e. g. list the used version of the RCU library in the manual, it would be nice to be able to read out the version from the sources. Many libraries have a define with a version string. This is a simple way to implement this.

I added such a define in a new header file urcu/urcu_version.h which is included in urcu/urcu.h for version 0.15.3. Please find attach the patch for this.

I look forward if this would be added into the main branch of urcu.

Thank you.