build: change cdrkit location in build-process.md

The cdrkit.org domain expired.
Thanks to gdm85 on IRC for reporting this.

Rebased-From: 27fc5277f73e510c2150dc29308fdf2dc6a96053
build: fix release name strings for gitian builds

When building from a distdir as gitian does, checking for the .git dir
is not reliable. Instead, ask git if we're in a repo.

Rebase this into 0.9.3 branch after final to make sure that 0.9.4 will
have correct version strings in rcs.

Rebased-From: c65cc8c