Bug 106439 - dmake: whitespace required before colon

dmake 4.12 on the below makefile.mk fails with "`b:' not found, and can't be
made" as it apparently mis-parses the "{$X}:" part ("{$X} :" would work, but
dmake/man/dmake.nc states that "[the] rule operator [...] does not require white
space to precede or follow it").

```
X = a b

Y .PHONY : $X

a b :
        touch $@

{$X}: makefile.mk
```
