John Ericson authored Sep 03, 2019 and Marge Bot committed Sep 05, 2019

Some where using `True` / `False`, a legacy of when they were in
`Config.hs`. See #16914 / d238d306 for a similar problem.

Also clean up the configure variables names for consistency and clarity
while we're at it. "Target" makes clear we are talking about outputted
code, not where GHC itself runs.