replace hardcoded /run with RUN_DIR macro

to use it, pass -DRUN_DIR="/var/run" in your CFLAGS.
optimally this should be set by autoconf to a default value if not
specified, and allowed to be overridden by a configure option,
for example --rundir=/var/run .

adresses #76

someone that knows his way around autoconf should look at the autoconf part of this, thanks.