can't pass both -b/-c and -r to INSTALL

[z-erica]

plan9port/INSTALL

Lines 6 to 24 in be7c68f

	case "x$1" in
	x)
	;;
	x-b)
	dobuild=true
	doinstall=false
	;;
	x-c)
	dobuild=false
	doinstall=true
	;;
	x-r)
	shift
	PLAN9_TARGET=$1
	;;
	*)
	echo 'usage: INSTALL [-b | -c] [-r path]' 1>&2
	exit 1
	esac

this block only runs once per invocation