This GNU make feature is either unsupported or broken in biomake.

Simple assignments throw parse error. Recursive assignments are parsed but have no effect.

Makefile:

main-target: variable = defined

main-target: dependency dependency-with-directly-defined-variable
	cat $^ > $@
	
dependency:
	echo "[" $(variable) "]" > $@

dependency-with-directly-defined-variable: directly_defined_variable = directly_defined

dependency-with-directly-defined-variable:
	echo "[" $(directly_defined_variable) "]" > $@

main-target built with GNU make:

[ defined ]
[ directly_defined ]

main-target built with biomake:

[ ]
[ ]

Since the README lists "various ways of setting variables" among supported features, and doesn't mention target-specific variable values among unsupported features, I would expect this feature to work.