Fixes so that QupZilla can run on Haiku

Compiles and runs, but crashes a lot due to a buggy QtWebKit port.
See commit messages for expanded summary.
I'm afraid not. Without linking the main app to execinfo, it fails the link because the symbols from execinfo are not defined (they are used in main.cpp, right?)

I highly doubt this builds on the *BSD systems either, but I don't have one around to test on...