>>23 ENVIRONMENT VARIABLES
The AWKPATH environment variable can be used to provide a list of
directories that gawk searches when looking for files named via the -f
and --file options.
If POSIXLY_CORRECT exists in the environment, then gawk behaves exactly
as if --posix had been specified on the command line. If --lint has
been specified, gawk issues a warning message to this effect.
the gnu diff man page doesn't even have an ENVIRONMENT VARIABLES section.