change RECIPEPREFIX to space work only with simple expansion

[lijh.8]

Hi community,


I tried to change RECIPEPREFIX to space. 
It works with simple variable expansion but not recursive expansion,
as shown in the first three assignments to it.
I ever thought that the simple expansion gurantees that it happens 
before enter recipe.


But in the manual 6.14, it presents an example with greater than sign ` > 
`, 
and it works with both simple and recursive expansion.


Thanks




```


null =
# .RECIPEPREFIX := $(null) # this comment keeps the space
# .RECIPEPREFIX := $' '
.RECIPEPREFIX := $(subst ,, )
# .RECIPEPREFIX = >
# .RECIPEPREFIX := >


all: a b c


a:
        @echo "a:" # tab indent


b:
 @echo "b:" # space indent


c:
> @echo "c:"


```