message: build: Add var printing target to src/Makefile.am
  See for more info. I missed
  this one last time.
diff: # 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)
  