[bug #64746] Exponential Runtime in make 4.4.1 when export is used

[Till Backhaus]

Follow-up Comment #2, bug #64746 (project make):

That explanation was really helpful. I discovered the difference between
recursively expanded variables and simple expanded variables only after
posting this issue, but I was still confused why recursive expanded variables
would cause this behavior. 
 
[comment #1 comment #1:]
> Prefer simply expanded variables with $(shell).
> 
> Given that sush questions keep arising, maybe we should update NEWS with a
warning about recursively expanded variables in combination with $(shell).

Yes, I think that's a great idea.

I'd also like to add a little more context about my use case: I use this to
read an .env file and then define variables that are still undefined.

-include .env
export

BUILD_OS ?= $(shell uname -s)
...

Up until 4.4 this wasn't a problem because $(shell) didn't see exported 
variables. What I'd like to have in 4.4 would be "conditional simple 
expansion" from a new ?:= operator that works like this:

ifeq ($(origin VAR_1), undefined)
  VAR_1 := $(shell echo 1)
endif

Until then I can use the verbose form from above, or I could introduce 
another variable to do it in two lines:

VAR_1_ := $(shell echo 1)
VAR_1 ?= ${VAR_1_}




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