Migration to autotools

This is my first try to migrate the (linux) build system to autotools. Hopefully OSX will follow.
Until now qBittorrent depended on the qconf utility which is abandoned and not very flexible. The idea is to use Autotools as little as possible and leave the rest on qmake to decide. The script tries to find a suitable qmake and initializes the INCLUDE and LIBS vars depending on the arguments passed to the configure script. Then it writes out conf.pri (like qconf did) and runs qmake. qmake is responsible for outputting the Makefile. If I transfered everything on autoconf/automake it would be a huge effort which isn't necessary. Now we have the flexibility of qmake with the support of autotools. Autotools will not dissapear anytime soon.

@glassez
Can you test if the **Qt5** detection works on your linux systems?
I based mine on Debian sid.
SIDENOTE: I just remembered that I didn't really have qt5 installed on Debian. grrr. I am pretty sure that it works though(at least for debian based systems). The script queries pkg-config for the libdir variable of Qt5core and then appends qt5/bin/qmake.

@Gelmir
If you use non-Debian based distros can you check if the script works?

Guys, just test the script with no extra args first (apart from --prefix).
I couldn't understand what to do with some options:
1. Disabling the use of geoip-database. I don't know what I should output in the conf.pri. Probably this was a dead option on the previous configure script.
2. embedding geoip doesn't do anything at the moment
3. I don't even know how to test if the system has qtsingleapplication installed. Debian doesn't provide it as a package. I searched on the online repos of Fedora and Arch. They seem to have a package for it. I don't know if it provides a .pc file (pkg-config) or I should use another way to detect it. For the moment configure just adds `usesystemqtsingleapplication` to CONFIG. I suspect that it has something to do with qtsingleapplication.pri but I CANNOT understand that file. Has anyone any ideas? (This is about the `--with-qtsingleapplication=system` option)
4. I don't know if some options should exclude others. eg Does nox disable dbus?

This is created just so we can have a discussion and test things. Try and break the configure script within the limits mentioned above. Also if you find any functionality missing please comment.