QT GUI Mac/Linux Wiki

Hi,

small update for the wiki, because the folder structure changed?

https://github.com/andrewssobral/bgslibrary/wiki/Graphical-User-Interface:-QT

Section "On Linux/Mac"

```
git clone --recursive https://github.com/andrewssobral/bgslibrary.git

cd bgslibrary/gui/qt/build
cmake ..
make

cp bgslibrary_gui ../../../
cd ../../../

./bgslibrary_gui
```

and if you manually installed QT on mac with
```
brew install qt
```
the folder PREFIX Path is following:
```
cmake .. -DCMAKE_PREFIX_PATH=/usr/local/Cellar/qt/5.13.0/
```

Cheers
Hannes