${GENCMD} is used to generate only one output from file generation commands

which potentially have multiple output files, which confuses make(1).

This wrapper copies all involved files into /tmp, execute the command, and
copy the wanted file back to ${.OBJDIR}.  If this is expensive, consider to
rewrite the generation command to output files individually.

XXX bsd.own.mk is too bloated.

share/mk/bsd.own.mk

@@ -1,4 +1,4 @@
-#	$NetBSD: bsd.own.mk,v 1.603 2009/12/06 16:15:15 uebayasi Exp $
+#	$NetBSD: bsd.own.mk,v 1.604 2009/12/11 08:37:34 uebayasi Exp $
 
 .if !defined(_BSD_OWN_MK_)
 _BSD_OWN_MK_=1
@@ -976,6 +976,28 @@ MAKEDIRTARGET=\
 	}; \
 	_makedirtarget
 
+#
+# ${GENCMD} <generated name> <command and arguments>
+#	Extract only one output from generation command so that make rule is
+#       written straight and avoid race (duplicate generation).  <generated
+#       name> is usually ${.TARGET}; if you want foo.c from foo.y, yacc
+#       generates y.tab.c, so <generated name> is y.tab.c.  See bin/sh/Makefile
+#	for examples.
+#
+GENCMD=\
+	@_gencmd() { \
+		local _dst="$$1"; shift; local _src="$$1"; shift; local _gen="$$1"; shift; \
+		local _tmp=$$( ${TOOL_MKTEMP} -d /tmp/XXXXXX ); \
+		local _opwd=$$PWD; \
+		local _exit; \
+		cp $$_src $$_tmp; \
+		{ cd $$_tmp && eval $$@; _exit=$$?; cd $$_opwd; }; \
+		[ $$_exit = 0 ] && cp $${_tmp}/$${_gen} $$_dst; \
+		rm -fr $$_tmp; \
+		return $$_exit; \
+	}; \
+	_gencmd "${.TARGET}" "${.ALLSRC}"
+
 #
 # MAKEVERBOSE support.  Levels are:
 #	0	Minimal output ("quiet")