The new get_env() method that returns an EnvironmentVariables object
will be needed in next commit that will pass it to CustomTarget.
This has the side effect to use the proper os specific path separator
instead of hardcoding `:`. It is the obvious right thing to do here, but
has caused issues in the past. Hopefully issues have been fixed in the
meantime. If not, better deal with fallouts than keep doing the wrong
thing forever.
18bec0d3e3