make: Unclosed substitution for CONFIGURE_DIRS (] missing)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

It looks like the change introduced in pkgsrc/mk/configure/configure.mk 1.28 
confuses at least my ancient (NetBSD-4) make. Escaping the pound sign with 
a backslash seems to correct the problem:

diff -u -r1.29 configure.mk
--- configure/configure.mk      7 May 2019 19:36:44 -0000       1.29
+++ configure/configure.mk      1 Jul 2019 16:47:06 -0000
@@ -206,7 +206,7 @@
 .PHONY: do-configure-script
 do-configure-script:
 .for _dir_ in ${CONFIGURE_DIRS}
-.  if ${CONFIGURE_DIRS:[#]} != 1
+.  if ${CONFIGURE_DIRS:[\#]} != 1
        ${RUN} ${STEP_MSG} "Running "${CONFIGURE_SCRIPT:Q}" in "${_dir_:Q}
 .  endif
        ${RUN}${_ULIMIT_CMD}                                            \

I've no idea whether the escaped pund sign confuses modern versions of make.