rawtherapee: fix version information
The existing version logic dates to 68a38d6c0fac3 (PR 18368 on
September 25, 2016). But rawtherapee switched from hg to git
around 2015, so these cmake variables were wrong almost as soon
as they were set.
Without this patch, running
rawtherapee-cli --version
has the output
RawTherapee, version , command line.
With this patch:
RawTherapee, version 5.12, command line.
which is significantly better, although personally I'd prefer to also
have a git commit ID.