AC_INIT: quote invalid feature names

Changes:
   configure: error: invalid feature name: debug
to
   configure: error: invalid feature name: `debug '
to make it obvious if trailing space is the reason why a feature
name was rejected; similar to existing error messages elsewhere
about invalid shell variable names.

* lib/autoconf/general.m4 (_AC_INIT_PARSE_ENABLE2): Add quotes.
Reported by Noel Grandin.

Signed-off-by: Eric Blake <eblake@redhat.com>
maint: add to THANKS

* THANKS: Update.

Signed-off-by: Eric Blake <eblake@redhat.com>