Doesn't build on macOS

[athairus]

https://bugreports.qt.io/browse/QTBUG-50073

Qt 5.6+ have removed Qt5PlatformSupport pkg-config and CMake files. We relied on this library to have a way of getting at the scene graph's OpenGL context so we could directly turn VSync on or off. We'll have to re-work this...

One possible solution is to make 5.6 the mandatory supported version... it is LTS after all?

[athairus]

After carving out osxhelper.mm and the dependent internal header from the build system, here's how the versions compare:

• Qt 5.5.1: Compiles, won't run. We use QtQuick 2.6 which isn't available here
• Qt 5.6.2: Compiles, runs, crashes on exit (see below)
• Qt 5.7.1: Compiles, runs, crashes on exit (see below)
• Qt 5.8.0: Does not compile due to error in 5.8.0's distribution

Crash on exit with quazip

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   org.qt-project.QtCore           0x0000000107b63de3 QDateTime::~QDateTime() + 19
1   Phoenix                         0x000000010608bfe3 QuaZipNewInfo::~QuaZipNewInfo() + 83
2   Phoenix                         0x0000000106087475 QuaZipNewInfo::~QuaZipNewInfo() + 21
3   Phoenix                         0x0000000106087117 DebugHandler::archive() + 487
4   Phoenix                         0x0000000106088657 main + 2759
5   libdyld.dylib                   0x00007fff8cf1a255 start + 1