w32: Add windows subsystem variant of gpgconf

* tools/Makefile.am (gpgconf-w32): New target. Builds gpgconf with
subsystem windows.
* build-aux/speedo/w32/wixlib.wxs: Package it.

This allows us to create scripts that call gpgconf without opening
a console. Using subsystem windows is better then to just close
the console after start as it avoids the console flashing up.