Allow the path to the system source directory to be passed in to

Allow the path to the system source directory to be passed in to
newvers.sh.  Pass it in from include/Makefile.  If it isn't passed in,
fall back to the old logic of using dirname $0.

Using dirname $0 does not yield the path to the script if it was
sourced in from another script in another directory; you end up with
the parent script's path.  That was causing newvers.sh to look one
level below the FreeBSD src/ directory when building osreldate.h and it
may find something like a git or svn repo there that has nothing to do
with FreeBSD.

PR:		174422
Approved by:	re ()
MFC after:	2 weeks