INSTALL.sh: the -a option had no effect at all

INSTALL.sh: the -a option had no effect at all

The sed looked for the BRISK_AUTH_CONF define in Obj/auth.phh, where it has
never been: it lives in Obj/dbase_file.phh. sed did not find the file, wrote
the error in the middle of the other installation lines, and the installation
carried on, so -a was ignored without anybody noticing.

The define is now looked for where it actually is, and if it is not found the
installation says so instead of keeping quiet.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014M1jiEq9cHdE5SE5j6vFuE