Don't warn about emacs in SHELLOPTS when readline was not compiled in

[Felix Lechner via Bug reports for the GNU Bourne Again SHell]

Hi,

On GNU Guix a minimal Bash produced the warning

   bash: line 0: emacs: invalid option name

while SHELLOPTS was set to

   braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor

In a discussion on IRC, the discussion concluded that the warning was
confusing and that Bash should perhaps ignore "emacs" since readline
wasn't compiled in.  Thanks!

Kind regards
Felix Lechner