This way it works even if we have to walk up the
directory hierarchy in cases where we have no env
var to rely on.

Other Linphone stack packages can use this to generate a
gitversion.h from debian/changelog for correct version
information.

Update to upstream version '5.0.37'
with Debian dir ef50f13b39e74d45cbc4e6a1645ddf8bfa8c79ae

Closes: #974749

As requested by himself

Update to upstream version '4.4.13'
with Debian dir c95b988fcb7906e5db2553735adca405a107b8cb

Update to upstream version '4.4.0'
with Debian dir d88f407dcb9bd8bcaaf91eb8352172d11c9d9c4e

Update to upstream version '4.3.1'
with Debian dir 884ba29ddafde48a7c3c6f10b1ccb9e672e8c1dd

Update to upstream version '4.3.0'
with Debian dir c53308290b04aa03f8de7af2c2b098ae182510a8

Since bctoolbox is the only Debian packaging repo of the whole linphone
stack that is using the upstream git repo directly, switch to importing
upstream tarballs to make it consistent.

Together with "git branch upstream 0.6.0" this should do the trick.