How to update /usr/src/ using SVN and "make update"?

Taking the instruction from manpage make.conf(5) and setting  variable
SVN_UPDATE to YES, typing "make update" in /usr/src	 fails due to svn
can not be found. make.conf(5) also contains a variable "SUP" containing
the path for cvsup or cvsup and I'm desperately missing a similar
variable for subversion. Since subversion is a port, it might be so that
setting a variable in make.conf(5) conflicts with the BSD policy and
logic, but then I'd appreciate a hint like "you need to set the propper
environment variable for finding svn in the search path ...blablabla".
Using "make update" ends up in "svn not found". Typing manually "svn -r
HEAD" in /usr/src as it is shown when the make update fails works well,
since the shell's environment PATH variable knows /usr/local/bin to look
for.
I also file a PR not letting this getting forgotten, please close it, if
it is obsolete.

Regards,
O. Hartmann