Conditional info_TEXINFOS

[Kevin Ryde]

Would it be easy to let info_TEXINFOS be conditionally defined?

        if WANT_FOO
        info_TEXINFOS = foo.texi
        endif

I think it should create and distribute the info files unconditionally,
but only install (or uninstall) them under WANT_FOO.  Perhaps it'd
just be a case of splitting out a conditionalized $(INFO_DEPS) for the
latter rules.

The advantage would be say when a library and header file are
installed only under some configure option, that the documentation
could similarly conditionally installed.