-ERROR_IF_UNSET = $(if $(filter undefined, $(origin $(strip $(1)))), $(error Makefile $(strip $(1)) value not set))
-ERROR_IF_EMPTY = $(if $(strip $($(strip $(1)))), , $(error Makefile $(strip $(1)) option cannot be blank))
-ERROR_IF_NONBOOL = $(if $(filter Y N, $($(strip $(1)))), , $(error Makefile $(strip $(1)) option must be Y or N))
+SHELL = /bin/sh
+
+ERROR_IF_UNSET ?= $(if $(filter undefined, $(origin $(strip $(1)))), $(error Makefile $(strip $(1)) value not set))
+ERROR_IF_EMPTY ?= $(if $(strip $($(strip $(1)))), , $(error Makefile $(strip $(1)) option cannot be blank))
+ERROR_IF_NONBOOL ?= $(if $(filter Y N, $($(strip $(1)))), , $(error Makefile $(strip $(1)) option must be Y or N))