build: Add variable printing target to Makefiles

I kept finding myself needing these to debug our build system, since
they are innocuous and are very helpful they probably belong in the
codebase.

Source: John Graham-Cumming
https://www.cmcrossroads.com/article/printing-value-makefile-variable

Makefile.am

@@ -4,6 +4,10 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or https://www.opensource.org/licenses/mit-license.php .
 
+# Pattern rule to print variables, e.g. make print-top_srcdir
+print-%:
+	@echo $* = $($*)
+
 ACLOCAL_AMFLAGS = -I build-aux/m4
 SUBDIRS = src
 if ENABLE_MAN

depends/Makefile

@@ -1,5 +1,9 @@
 .NOTPARALLEL :
 
+# Pattern rule to print variables, e.g. make print-top_srcdir
+print-%:
+	@echo $* = $($*)
+
 SOURCES_PATH ?= $(BASEDIR)/sources
 BASE_CACHE ?= $(BASEDIR)/built
 SDK_PATH ?= $(BASEDIR)/SDKs