environmental variables are recursively expanded

[gk]

I have just discovered that environmental variables are recursively expanded.
I did not see any mention of this in the manual and I think it is worthy of 
note.
--- Makefile
a+=$(a)
foo:
        @echo $(a)
---

address@hidden junk]$ export a=pre; make
Makefile:3: *** Recursive variable `a' references itself (eventually).  Stop.

Is this behavior by accident or design?
I was expecting (hoping) that they would be simply expanded by default 
since this is more often what one wants.

- Greg Keraunen