# check_defined is a function that will error if the passed in variable is
# undefined.
#
# usage:
#   install:
#     $(call check_defined, var1)
#     $(call check_defined, var2)
#
# Copied this from https://gist.github.com/bbl/bf4bf5875d0c705c4cd78d264f98a8b1
check_defined = \
    $(strip $(foreach 1,$1, \
        $(call __check_defined,$1,$(strip $(value 2)))))
__check_defined = \
    $(if $(value $1),, \
        $(error Undefined $1$(if $2, ($2))))