Introduce -DBUILD_WITH_DPKG to toggle dpkg based install

The current system automagically chooses between the dpkg-based install method
(for debian-derived distributions) and a "traditional" install and sets
install paths based on whether cmake can find a 'dpkg' executable.

This is not ideal, since dpkg is occasionally installed on linux distributions
unrelated to debian for purposes other than package management. In particular,
Arch and Gentoo carry it in their repositories.
don't dpkg on windows and macos

No need to clutter windows and macos configuration with dpkg options, as it is
unlikely to ever be useful.
Merge remote-tracking branch 'scrawl/master'
Merge remote-tracking branch 'scrawl/crashcatcher'
Closing message boxes with numpad enter
fixed expression in if statement
Merge branch 'master' of https://github.com/zinnschlag/openmw