speedbar should recognise imenu support better

[Kevin Broadey]

In GNU Emacs 20.6.1 (sparc-sun-solaris2.6, X toolkit)
 of Mon Jul  3 2000 on bah108
configured using `configure  --prefix=/tools.local/emacs-20.6 
--bindir=/tools.local/emacs-20.6/bin/SunOS --with-x-toolkit=lucid'

The auto-mode-alist variable associates *.mk files with makefile-mode,
but if I set speedbar-show-unknown-files then I see a [?] against the
file, even though imenu indexing is available for it.

I think speedbar should use the major mode for each file to decide
whether it can index it, rather than relying on a filename regexp as
it does at present.

--
Kevin Broadey