add a "show-indexed" target that displays variables with (potential)

SUBPACKAGE variation.

Less brittle than make show=FULLPKGNAME\${SUBPACKAGE}

infrastructure/mk/bsd.port.mk

@@ -1,6 +1,6 @@
 #-*- mode: Makefile; tab-width: 4; -*-
 # ex:ts=4 sw=4 filetype=make:
-#	$OpenBSD: bsd.port.mk,v 1.1458 2018/12/12 12:04:56 espie Exp $
+#	$OpenBSD: bsd.port.mk,v 1.1459 2018/12/12 16:17:30 espie Exp $
 #
 #	bsd.port.mk - 940820 Jordan K. Hubbard.
 #	This file is in the public domain.
@@ -3477,6 +3477,15 @@ show:
 	@echo ${${_s}:Q}
 .endfor
 
+show-indexed:
+.for _s in ${show-indexed}
+.  if defined(${_s}${SUBPACKAGE})
+	@echo ${${_s}${SUBPACKAGE}:Q}
+.  else
+	@echo ${${_s}:Q}
+.  endif
+.endfor
+
 # du fails if it can't access everything
 show-size:
 	@${_PBUILD} du -ks ${WRKDIR}