Use a more secure method of creating the temporary

install directory.

Makefile.inc1

@@ -140,9 +140,7 @@ STRICTTMPPATH=	${WORLDTMP}/usr/sbin:${WORLDTMP}/usr/bin:${WORLDTMP}/usr/games
 TMPPATH=	${STRICTTMPPATH}:${PATH}
 OBJFORMAT_PATH?=	/usr/libexec
 
-TMPDIR?=	/tmp
-TMPPID!=	echo $$$$
-INSTALLTMP=	${TMPDIR}/install.${TMPPID}
+INSTALLTMP!=	mktemp -d -u -t install
 
 #
 # Building a world goes through the following stages