In GNU Make, how do I convert a variable to lower case?

you can always spawn off tr

LOWER_VAR = `echo $(VAR) | tr A-Z a-z`

or

LOWER_VAR  = $(shell echo $(VAR) | tr A-Z a-z)

The ‘lc’ functions you trying to call is from GNU Make Standard Library

Assuming that is installed , the proper syntax would be

LOWER_VAR  = $(call lc,$(VAR))