Andrey Mokhov authored Jun 23, 2019 and Marge Bot committed Jun 25, 2019

This resolves #16809 (ghc/ghc#16809).

This patch removes the unnecessary dependency on configure-generated
flags `windowsHost`, `osxHost` and `iosHost`, using the information
provided by the module `System.Info` instead.

We also take care to use the `CrossCompiling` flag generated by the
configure script only after the latter had a chance to run.