Second try: qmake integration alongside normal build

After some fiddling, I found a way to make the qmake files coexist with the regular makefiles, so they do not need to be removed and do not impact the project otherwise.

Since flexbuf.c and preprocess.c are already being built as cpp files, I changed their extension to reflect that, and so that qmake would compile them correctly (the other option was declaring the headers as extern "C").

I've also added a top-level LICENSE file since the project was missing one.