Request for clarification on menu/file spec

[Jeremy White]

> If you choose an /opt prefix, the .desktop file should be in
> /opt/<app-name>/share/applications

Gah!!!!  You are not understanding.

I am giving the *user* the choice of where to install
the software to.  They can pick /blamblam if they
feel like it.

And if they do pick /blamblam, it is completely
unreasonable to say that the user must now
add /blamblam to their XDG_DATA_DIRS.  (Even if
that were the requirement, there is no spec that
tells me how and where to set XDG_DATA_DIRS, so I
cannot do that reliably).

It is a very simple thing I wish to accomplish:
I wish to, programatically, cause a menu entry
to appear in a users favorite XDG menu spec compliant
window manager.

And the menu-spec is achingly close to giving
me a nice spec for that:
  http://standards.freedesktop.org/menu-spec/latest/apc.html

It works fine for user installs, for example, because
'datadir' and 'sysconfdir' have exactly one possible value
for user installs, and an exact algorithm locates them.

But for root installs, there is no specification that
describes how to exactly determine what 'datadir'
and 'sysconfdir' should be; that is the logical hole
I am trying to fill.

Cheers,

Jeremy