Prefer stack installed GHC even if it's the same as the system GHC

[mgsloan]

See e.g. the issue here: #1728 (comment)

[mgsloan]

Some discussions of why the system ghc is preferred:

• https://groups.google.com/forum/#!topic/haskell-stack/-XaXiq5Wdo8
• Change default to not use system-ghc unless otherwise specified. #1129

I'm not sure if I buy it, I'm leaning towards making "system ghc" usage opt-in, but this would be a breaking change...

[sjakobi]

Closing this in favor of the more decided #2221.