The makefile-mode is not general enough to make use of Plan 9's user
space mkfile format and mk command.

This is likely to be considered not a bug.  The expected behavior is
Emacs would be smart with Plan 9's user space tools.  Specifically,
the mk command and mkfile format in this case.

The unexpected behavior attempts to compile using the make command and
not the mk command.  I tried to locate a PLAN9Makefile-mode but didn't
find one in the command completion mechanism.

steps to reproduce
 - set PLAN9 and PATH environment variables in shell
 - verify, mk all, works at the plain shell
 - start, emacs -Q
 - visit /tmp/mkfile; makefile-mode does not recognize mkfile
 - M-x makefile-mode
 - M-x compile RET; Compile command: make -k, should adapt to mk command

quote
# example /tmp/mkfile
all:V:
  echo This line is left blank intentionally.
  
quote ends