undocumented variable MAKE_COMMAND

[Frank Heckenbach]

I got some strange make errors about resetting jobserver mode etc.

Long story short, after some debugging I found the reason:

I use an optional wrapper for make which I invoke via an environment
variable which I (plausbly) named MAKE_COMMAND. To my surprise, I
found that recursive makes using $(MAKE) called my wrapper instead
of make.

Apparently make reads this variable, but this is not documented.

Now that I grepped for it, I see that ChangeLog mentions it in the
context of VMS in 2014, and a NEWS entry from 1993 says:

  * The `MAKE' variable is always defined as `$(MAKE_COMMAND) $(MAKEOVERRIDES)'.
    The `MAKE_COMMAND' variable is now defined to the name with which make
    was invoked.

None of these can really count as current documentation anyway, but
even if I had seen it, I would have understood it to mean that make
sets (in my case, overrrides) the variable before it uses it.

Most importantly, the manual doesn't mention it at all. I think it
should prominently mention a variable that has such a profound
impact on make's operation.