Ubuntu packaging: add support for better changelog tracking

This now assumes that a running changelog is maintained in
src/debian.changelog, i.e., at the same level as the subsurface tree; the
organization now should look like this:

src/debian.changelog
src/subsurface                 # subsurface git checkout
src/subsurface/libdivecomputer # libdivecomputer git Subsurface-xx branch
src/subsurface/marble-source   # marble git Subsurface-xx branch
src/subsurface/libgit2         # libgit2 git checkout

Instead of running dh_make to create all new debian build files, we add the
necessary files in our script.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Ubuntu packaging: more tweaks

* only build a static libdivecomputer
* only build the libgit2 library, not the executable
* don't echo all the symlinks when fake-installing libmarblewidget

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Document the use of SPECIAL_MARBLE_PREFIX=1

Since we suggest building with our marble branch we need to tell people to use
SPECIAL_MARBLE_PREFIX=1 or the build process won't find the correct library.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>