Merge bitcoin#17087: build: Add variable printing target to Makefiles

181989f build: Add variable printing target to Makefiles (Carl Dong)

Pull request description:

  ```
  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
  ```

ACKs for top commit:
  MarcoFalke:
    ACK 181989f
  fanquake:
    ACK 181989f - concise amount of useful code. Tested on macOS. Did not visit the link.

Tree-SHA512: 2139621e68a499c7347663ca9dc04e166ea6280e05986c27858df0156016ef2f9461262464d70c601419384f43a4ae3bcc67dfc0a05dbeef64f08386ab429cd8

Author: fanquake

Makefile.am

@@ -3,6 +3,10 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://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
 WORK_PATH = $(BASEDIR)/work
 BASE_CACHE ?= $(BASEDIR)/built