Support Qt5 build and install

kdesrc-build used to support building Qt in addition to KDE. This changed with Qt 5 since it was a significantly different build system and most distributions packaged a recent enough Qt to permit using distro devel packages.

However we now track to only requiring the current or 2 previous minor releases of Qt, which is often more recent than what popular stable or LTS distros might provide. Debian in particular, see #15 (closed) for an example.

To account for this kdesrc-build should support building, installing, and using git versions of Qt 5. Or at the very least tarball releases. There is a highly experimental way to do this with existing code that is floating around, but a dedicated build system using Qt 5's custom build framework seems more future-proof.