Why can't we override make program?

mk/bsd.pkg.mk

.if defined(_USE_GMAKE)
MAKE_PROGRAM=           ${GMAKE}
.elif defined(USE_IMAKE)
MAKE_PROGRAM=           ${_IMAKE_MAKE}
.else
MAKE_PROGRAM=           ${MAKE}
.endif

Replacing the last line '=' with '?=' would allow me to override
MAKE_PROGRAM with ${PREFIX}/bin/dxmake for backplane database I am preparing
for -wip.

Take care,
-- 
Michal Pasternak :: http://pasternak.w.lub.pl :: http://winsrc.sf.net