src: Simplify installing/uninstalling data

src: Simplify installing/uninstalling data

Instead of defining targets conditionally and depending on
them unconditionally, define a couple of variables and
conditionally add targets to them.

In addition to removing a bunch of useless code, this has
the nice effect of no longer requiring the main Makefile.am
to have any knowledge about the contents of the various
snippets it includes.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>