The patch in comment #2 changing only the case of "host" suggests the all-caps spelling may have been used when a coder was misled by the reportedly (in bug #64913) incorrect casing of that variable's documentation in Makefile.am.

In my latest groff build (a few months old, but I don't think the build system has changed significantly if at all), the entirety of the charset.alias file is:

# This file contains a table of character encoding aliases,
# suitable for operating system ''.
# It was automatically generated from config.charset.
# Packages using this file:

So clearly something is going wrong, the operating system string being left empty.  Tellingly, on my system the environment variable $HOST is undefined, suggesting that the make rule building charset.alias is using environment variable $HOST where it means to use make variable $host.

The comment #1 paragraph about localcharset replacing config.charset strikes me as a separate issue.

From baf66e7e75a5332f311f043fab849fe7e668779d Mon Sep 17 00:00:00 2001
From: Bjarni Ingi Gislason <bjarniig@simnet.is>
Date: Fri, 24 Nov 2023 22:03:00 +0000
Subject: [PATCH] libgroff.am: "charset.alias" needs the variable
 'host' (defined) instead of 'HOST'

Directory "src/libs/libgroff".

  Variable 'host' is defined by "automake" in "Makefile.in".

Signed-off-by: Bjarni Ingi Gislason <bjarniig@simnet.is>
---
 src/libs/libgroff/libgroff.am | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libs/libgroff/libgroff.am b/src/libs/libgroff/libgroff.am
index 8dbccecd4..1a50a6991 100644
--- a/src/libs/libgroff/libgroff.am
+++ b/src/libs/libgroff/libgroff.am
@@ -125,7 +125,7 @@ all: charset.alias ref-add.sed ref-del.sed

 charset.alias: $(libgroff_srcdir)/config.charset
         $(AM_V_GEN)$(SHELL) $(libgroff_srcdir)/config.charset \
-            '$(HOST)' > t-$@ \
+            '$(host)' > t-$@ \
           && mv t-$@ $@

 ref-add.sed: $(libgroff_srcdir)/ref-add.sin
--
2.42.0

  The patch is (also) in the attachment.


(file #55353)

Subject: libgroff.am: charset.alias is created (almost empty) with
wrong value of "$HOST"

  "charset.alias" is created in

  src/libs/libgroff/libgroff.am

  with

charset.alias: $(libgroff_srcdir)/config.charset
        $(AM_V_GEN)$(SHELL) $(libgroff_srcdir)/config.charset \
            '$(HOST)' > t-$@ \
          && mv t-$@ $@

Content is:
---cut---
# This file contains a table of character encoding aliases,
# suitable for operating system 'kassi'.
# It was automatically generated from config.charset.
# Packages using this file:
---cut---

  "kassi" is the value of the environmental variable HOST, which is the
name of the computer.

  It is documented in the Makefile as being

# HOST
# `HOST' is the canonical host specification,
#    CPU_TYPE-MANUFACTURER-OPERATING_SYSTEM
# or
#    CPU_TYPE-MANUFACTURER-KERNEL-OPERATING_SYSTEM

  and should therefore have the same content as

build_triplet = x86_64-pc-linux-gnu
host_triplet = x86_64-pc-linux-gnu

build = x86_64-pc-linux-gnu

host = x86_64-pc-linux-gnu

  or

host_cpu = x86_64
host_os = linux-gnu
host_vendor = pc

  The file "config.charset" is from GNU GNULIB, which removed it

2018-05-19  Bruno Haible  <bruno@clisp.org> (in Changelog)

        * lib/config.charset: Remove file.