Add qmake for buildsystem public stack

This should complete our buildsystem stack, allowing downstream users to junction and be able to use any BuildStream build-element plugin.

Qt-base builds qmake during the configure script, so this is used for the build commands. My initial approach was to build qmake using Makefile.unix in the qmake/ directory, but this proved an issue due to a load of configuration done before using this makefile. Looking at source code seemed to depend on a lot of qt dependencies, so I wrongfully assumed that there were some dependencies on the qt core, but I think just running the configure script is sufficient. I should probably have noticed this earlier.

The options passed to configure are mainly to stop it checking for dependencies we don't have staged, in particular without -no-gui it will check for openGL.