For now we extract the exact version string of the ppa build
(which differs from our usual version string).

This should be matched to other builds, though, if somebody can
figure it out.

'dist' is the same as before

only difference:
add date and commit hash for development and pre-release

The idea behind inkscape-version.h / inkscape-version.cpp is to
avoid rebuilding most of the code base when the version number
changes. Defining a version number in config.h defeats this purpose.

(re-use inkscape-version.cpp in order not to duplicate code)

(they don't have a '.git/' directory, but only a '.git' file storing the link to the gitdir)