Drop redundant `-D__GLASGOW_HASKELL__=...` flag
In 3549c952 a `include/ghcversions.h` include file was introduced which defines `__GLASGOW_HASKELL__` as well. So there's no need to define it twice.
In 3549c952 a `include/ghcversions.h` include file was introduced which defines `__GLASGOW_HASKELL__` as well. So there's no need to define it twice.