Set the correct executable path when installing bsnes.desktop

[Screwtapello]

In POSIXy environments, make install installs a bsnes.desktop file that (among other things) says:

Exec=bsnes

That is, "to execute this application, launch the first bsnes executable on the default $PATH". However, we have no guarantee that our install destination is on the default $PATH (and on many Linux distros, it isn't) so this doesn't work as well as it should.

Instead of installing the file with cp, we should use sed at install time to set the actual path we installed the binary to, just to be sure it works.

[Pobega]

There's also the deskto-file-edit command, like so:

desktop-file-edit --set-key=Exec --set-value="${PREFIX}/bsnes" /path/to/bsnes.desktop

I think this is preferred over using sed to edit desktop files.

(sorry for random comment, just dropping in from github / topics / emulators 👍 )

Though I guess it's more likely the host has sed than desktop-file-edit so maybe it's a pointless suggestion