kmk: Add .RETURN and .ARGC to $(evalcall ) and $(evalcall2 ) for creating functions

[bird]

To create more complicated functions without also creating an unreadable mess, it's necessary to make $(evalcall[2] ) be able to return a value. This is implemented using a local variable .RETURN, the function body assigns the desired return value to this variable.

Example:

define POP
local words := $(words $($1))
local .RETURN := $(word $(words), $($1))
$1 := $(wordlist 1, $(expr $(words) - 1), $($1))
endef

stack-var = a b c d
d-element := $(evalcall POP,stack-var)

In addition, it's very useful to have some way of getting at the argument count without evaluating all the possibilities. For this purposed .ARGC was added. It is also applied to $(call ).