make delete-old broken

Hello.
In the interval between revisions >= r254887 to r255016
make delete-old broken

root_at_nonamehost:/ # cd /usr/src/
root_at_nonamehost:/usr/src # yes|make delete-old
make[1]: "tools/build/mk/tools/build/mk/OptionalObsoleteFiles.inc" line
1603: Malformed conditional (${MK_GNU_PATCH} == no) make[1]: Fatal
errors encountered -- cannot continue make[1]: stopped in /usr/src
*** Error code 1

Stop.
make: stopped in /usr/src

I can not say exactly at what revision it happened - but it was noticed
in revision r255016