Confusing error message in development version

[Gavin Smith]

Git revision a9aacb0b39967059b8064

If I use a non-existent customization variable, like

./texi2any.pl  test.texi -c OPEN_CODE_QUOTE=\`

the error message is

texi2any: warning: GNUT_set_from_cmdline: unknown variable OPEN_CODE_QUOTE

The reference to GNUT_set_from_cmdline is confusing so whatever this is it
should be removed from the error message.  I'll look into it myself if I
have time.