don't let the make spec determine the host mode any more

don't let the make spec determine the host mode any more

(some of) the generators are (moderately) x-platform, so it makes no
sense to nail the host platform to a generator (and thus a spec).
overriding the host platform is only a debugging mesasure anyway, so one
can use the (now undocumented) -unix/-macx/-win32 options for that.

Change-Id: If2a059f1feeb2c726e5838625ede1c7add829985
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>