+# Helper variables.
+_empty =
+_sp = $(_empty) $(_empty)
+
+# _equal,S1,S2
+# ------------
+# If S1 == S2, return S1, otherwise the empty string.
+_equal = $(and $(findstring $(1),$(2)),$(findstring $(2),$(1)))
+
+# member-check,VARIABLE,VALID-VALUES
+# ----------------------------------
+# Check that $(VARIABLE) is in the space-separated list of VALID-VALUES, and
+# return it. Die otherwise.
+member-check = \
+ $(strip \
+ $(if $($(1)), \
+ $(if $(findstring $(_sp),$($(1))), \
+ $(error invalid $(1): '$($(1))', expected $(2)), \
+ $(or $(findstring $(_sp)$($(1))$(_sp),$(_sp)$(2)$(_sp)), \
+ $(error invalid $(1): '$($(1))', expected $(2)))), \
+ $(error $(1) undefined)))
+