When compiling C++ sources and BUILDSEED is set, pass down

-frandom-seed with a reduced version of .ALLSRC and .TARGET
as argument.  This allows bit-identical builds of C++ files.

share/mk/bsd.README

@@ -1,4 +1,4 @@
-#	$NetBSD: bsd.README,v 1.216 2008/07/29 04:40:14 mrg Exp $
+#	$NetBSD: bsd.README,v 1.217 2008/08/04 16:02:25 joerg Exp $
 #	@(#)bsd.README	8.2 (Berkeley) 4/2/94
 
 This is the README file for the make "include" files for the NetBSD
@@ -1267,6 +1267,13 @@ AS		Assembler.  [as]
 AFLAGS		Options to ${CC} when compiling or linking .s or .S
 		assembly source files.  []
 
+BUILDSEED	GCC uses random numbers when compiling C++ code.
+		If this option is present, seed the random number
+		generator based on the value, source file names and
+		the output file name to make builds more deterministic.
+		Additional information is available in the GCC
+		documentation of -frandom-seed.
+
 CC		C compiler.  [cc]
 
 CFLAGS		Options to ${CC}.  [Usually -O or -O2]

share/mk/sys.mk

@@ -1,4 +1,4 @@
-#	$NetBSD: sys.mk,v 1.97 2008/05/06 22:19:11 uwe Exp $
+#	$NetBSD: sys.mk,v 1.98 2008/08/04 16:02:25 joerg Exp $
 #	@(#)sys.mk	8.2 (Berkeley) 3/21/94
 
 unix?=		We run NetBSD.
@@ -51,7 +51,13 @@ LINK.c?=	${CC} ${CFLAGS} ${CPPFLAGS} ${LDFLAGS}
 CXX?=		c++
 CXXFLAGS?=	${CFLAGS:N-Wno-traditional:N-Wstrict-prototypes:N-Wmissing-prototypes:N-std=gnu99}
 
-COMPILE.cc?=	${CXX} ${CXXFLAGS} ${CPPFLAGS} -c
+__ALLSRC1=	${empty(DESTDIR):?${.ALLSRC}:${.ALLSRC:S|^${DESTDIR}|^destdir|}}
+__ALLSRC2=	${empty(MAKEOBJDIR):?${__ALLSRC1}:${__ALLSRC1:S|^${MAKEOBJDIR}|^obj|}}
+__ALLSRC3=	${empty(NETBSDSRCDIR):?${__ALLSRC2}:${__ALLSRC2:S|^${NETBSDSRCDIR}|^src|}}
+
+_CXXSEED?=	${BUILDSEED:D-frandom-seed=${BUILDSEED:Q}/${__ALLSRC3:O:Q}/${.TARGET:Q}}
+
+COMPILE.cc?=	${CXX} ${_CXXSEED} ${CXXFLAGS} ${CPPFLAGS} -c
 LINK.cc?=	${CXX} ${CXXFLAGS} ${CPPFLAGS} ${LDFLAGS}
 
 OBJC?=		${CC}