Code with minor changes taken from Chris Kelly blog. I just have added Makefile and sql script to register extesion.
To compile the source code postgresql-devel package is required, to fullfill requirement for RedHat/CentOS do yum install postgresql-devel, for Debian/Ubuntu you should install postgresql-server-dev package (sudo apt-get install postgresql-server-dev):
make install
CREATE EXTENSION pg_setpriority;
SELECT setpriority(10 /* priority integer */);
SELECT getpriority(/* pid integer default 0 */);