Use GHC version for coercion rules

Using the library version didn't make much sense, especially since the
tests-ghc tests had to switch on compiler version anyway, but also
because compiling without cabal would prevent the code from being used.
The conditional fake MIN_VERSION_base definition should probably stay up
top where I moved it, though, in case someone needs to use it to adjust
imports or exports in the future--the top seems an inherently better
place for that.