build-sys: add a rule to print a variable

$ make print-CFLAGS
CFLAGS=-fsanitize=address -Og -g

Trick from various sources:
https://stackoverflow.com/questions/16467718/how-to-print-out-a-variable-in-makefile
https://www.cmcrossroads.com/article/printing-value-makefile-variable

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180104160523.22995-4-marcandre.lureau@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -8,9 +8,12 @@ SRC_PATH=.
 
 UNCHECKED_GOALS := %clean TAGS cscope ctags dist \
     html info pdf txt \
-    help check-help \
+    help check-help print-% \
     docker docker-% vm-test vm-build-%
 
+print-%:
+	@echo '$*=$($*)'
+
 # All following code might depend on configuration variables
 ifneq ($(wildcard config-host.mak),)
 # Put the all: rule here so that config-host.mak can contain dependencies.

docs/devel/build-system.txt

@@ -510,3 +510,16 @@ default-configs/$TARGET-NAME file as input.
 This is the entrypoint used when make recurses to build a single system
 or userspace emulator target. It is merely a symlink back to the
 Makefile.target in the top level.
+
+
+Useful make targets
+===================
+
+- help
+
+  Print a help message for the most common build targets.
+
+- print-VAR
+
+  Print the value of the variable VAR. Useful for debugging the build
+  system.