Undo my personal disabling of the Qt download/build/install.

I did that because I didn't want to wait a day for Qt to build, but
didn't intend to make that change in the official source.

In the longer term, we should perhaps change the script to download and
install a binary package; sadly, their installers can't be made 100%
command-line and automated, but, well, the CMake installer pops up a
dialog, too, and if you don't have Java installed, some of the configure
scripts that check for Java pop up non-blocking "do you want to install
Java?" dialogs from OS X.

Change-Id: I99781d5e54529955bf9363a7ee9d4122403fa955
Reviewed-on: https://code.wireshark.org/review/5032
Reviewed-by: Guy Harris <guy@alum.mit.edu>