make the windows configure also use config_help.txt

specifically, make configure.bat dump the text file (which got some
windows-specific adjustments).

incidentally, this change removes the need for including a pre-built
configure.exe into our source packages.

Change-Id: Ib3515c113f3602767554fe1493df226551a7bf10
Reviewed-by: Lars Knoll <lars.knoll@qt.io>