Auto merge of #24315 - richo:make-debug-helper, r=pnkfelix

Also add docs because not being able to discover these things is the pits.

This was lifted from http://blog.jgc.org/2015/04/the-one-line-you-should-add-to-every.html , and solves a problem I've solved myself a *lot* of times, often in massively inferior ways.

Super open to documenting it somewhere other than nitty-gritty, which isn't immediately obvious.

Makefile.in

@@ -159,6 +159,13 @@
 #
 # Admittedly this is a little convoluted.
 #
+# If you find yourself working on the make infrastructure itself, and trying to
+# find the value of a given variable after expansion, you can use:
+#
+# make print-VARIABLE_NAME
+#
+# To extract it
+#
 # </nitty-gritty>
 #
 

mk/util.mk

@@ -16,4 +16,7 @@ else
   E = echo $(1)
 endif
 
+print-%:
+	@echo $*=$($*)
+
 S := $(CFG_SRC_DIR)