+#################### convenience, for debugging only
+# make +foo : prints the value of $(foo)
+# make ++foo : idem but verbose, i.e. foo=$(foo)
+++%: varname=$(subst +,,$@)
+++%:
+ @echo "$(varname)=$($(varname))"
++%: varname=$(subst +,,$@)
++%:
+ @echo "$($(varname))"