Windows: Make DESTDIR work

DESTDIR can't be used on Windows the same way as on Unix, the device
part of the installation paths get in the way.

To remedy this, have INSTALLTOP, OPENSSLDIR and ENGINESDIR get
different values depending on if $(DESTDIR) is empty or not, and use
$(INSTALLTOP), $(OPENSSLDIR) and $(ENGINESDIR) alone.

Reviewed-by: Andy Polyakov <appro@openssl.org>