bin/57029: make(1) undocumented builtin variables

[hopkinza%gmail.com@localhost]

>Number:         57029
>Category:       bin
>Synopsis:       make(1) undocumented builtin variables
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    bin-bug-people
>State:          open
>Class:          doc-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Sep 27 20:30:01 +0000 2022
>Originator:     Zach Hopkins
>Release:        NetBSD 9.3
>Organization:
n/a
>Environment:
NetBSD netbsdvm 9.3 NetBSD 9.3 (GENERIC) #0: Thu Aug 4 15:30:37 UTC 2022 mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64
>Description:
Some builtin variables such as .MAKE.OS are not mentioned in the manual.

Others include MACHINE and MACHINE_ARCH, which are only mentioned under the ENVIRONMENT section: "uses the following environment variables, if they exist: MACHINE, MACHINE_ARCH, ...". However, looking at usr.bin/make/main.c, make will indeed set them itself, at least in some circumstances.

There may be others I am unaware of as well.
>How-To-Repeat:
n/a
>Fix:
n/a