Test for make supporting abspath, and fail if it doesn't

This fixes problems using GNU make 3.80.

testsuite/mk/boilerplate.mk

@@ -5,7 +5,11 @@ HAVE_EVAL := NO
 $(eval HAVE_EVAL := YES)
 
 ifeq "$(HAVE_EVAL)" "NO"
-$(error Your make does not support eval. You need GNU make >= 3.80)
+$(error Your make does not support eval. You need GNU make >= 3.81)
+endif
+
+ifeq "$(abspath /)" ""
+$(error Your make does not support abspath. You need GNU make >= 3.81)
 endif
 
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