Tweaks for building/installing on Linux.

Hey John,

No code changes here, just a few added files and some changes to Sigil's main cmake file to make installing on linux a little easier. A launch script is created/configured that sets some env variables so a user can do the usual: cmake, make, sudo make install and then type sigil to run the program. I relocated the query_qmake function so linux could make use of it as well as Windows. I don't think I broke anything in doing so.

I also added a new custom target for linux that automates the building of a *.deb package containing a self-contained, distributable binary package by typing "sudo make makedebpkg". It will probably see limited use (maybe even just me!).

Let me know if you want something changed or eliminated, or if you don't want any of it.

Thanks,
Doug