Simplify CMake code to generate viewer_version.txt and summary.json.

The quoting used for the cygwin printf command didn't work for 'cmake -E
echo'. The whole content was enclosed in single quotes, with individual
string elements enclosed in double quotes. But we ended up with a summary.json
containing (e.g.):
'{Type:viewer,Version:3.7.26.33262}'
instead of the desired:
{"Type":"viewer","Version":"3.7.26.33262"}
HOWEVER: I see no compelling reason why either of these files must be deferred
to build time. It's simpler and more robust to generate them both directly
from CMake at configure time.