so far, qmake.exe was built in qmake/ and then copied to bin/, with
possible errors in the second step ignored. this made no sense.
this unifies the nmake makefile with the unix one; compare 46e51ce1d.
Change-Id: Ieb9c7cd46f0be0501d17e297808ac1cdad1b3c4a
Reviewed-by: Lars Knoll <lars.knoll@qt.io>