ax_have_qt: Autodetect Qt 5 by querying $PATH and qmake

The old logic for detecting Qt did not work for newer versions of Qt,
nor did it work on newer Debian/Ubuntu because of multiarch. I've
replaced the old hacks and heuristics with clean code that uses a Qt 5
pro file to detect Qt's parameters from $PATH and qmake. Qt 4 and
earlier do not support this kind of introspection, but any version of
Qt can still be used by explicitly specifying its parameters.