Generate ghcversion.h with the top-level configure --- contains !6833

This is, rather unintuitively, part of the goal of making the packages that make of the GHC distribution more freestanding. ghcversion.h is very simple, so we easily can move it out of the main build systems (make and Hadrian). By doing so, the RTS becomes less of a special case to those build systems as the header, already existing in the source tree, appears like any other.

We could do this with the upcomming RTS configure, but it hardly matters because there is nothing platform-specific here, it is just versioning information like the other files the top-level configure can be responsible for.