virtualbox: Explicitly state Qt 5 dependencies

In 29428159

, the dependencies for Qt 5
were passed using buildEnv with all the development binaries, headers
and libs. Unfortunately, the build output references that environment
which also increases the size of the runtime closure.

The upstream makefile assumes a common Qt 5 library path, but that's not
the case within Nix, because we have separate paths for the Qt 5
modules.

We now patch the makefile to recognize PATH_QT5_X11_EXTRAS_{LIB,INC} so
that we can pass in the relevant paths from Qt5X11Extras.

In summary, the closure size goes down to 525559600 bytes (501 MB)
instead of 863035544 bytes (823 MB) with vbox-qt5-env.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>