[PATCH 0 of 2 STABLE] Two minor wix tweaks

[Adrian Buehlmann]

(cc-ing mg as most promising crew pushing victim :-)

see individual patches

The build environment is already ready for the new build environment variable
introduced in the first patch. See
http://bitbucket.org/tortoisehg/thg-winbuild/changeset/315e649e161b/

I would like to keep things consistent and have these pushed to stable (and
then merged to the default branch).