build: Add var printing target to src/Makefile.am

See 181989f6c9 for more info. I missed
this one last time.

src/Makefile.am

@@ -2,6 +2,10 @@
 # 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 $* = $($*)
+
 DIST_SUBDIRS = secp256k1 univalue
 
 AM_LDFLAGS = $(LIBTOOL_LDFLAGS) $(HARDENED_LDFLAGS) $(GPROF_LDFLAGS) $(SANITIZER_LDFLAGS)