Here's full instructions on how to build the 'The Easy Way™' see here for why
Download http://download.qt.io/official_releases/qt/5.9/5.9.3/qt-opensource-linux-x64-5.9.3.run from the Qt website and follow the instructions below in terminal
cd ~/Downloads/
chmod +x qt-opensource-linux-x64-5.9.3.run
./qt-opensource-linux-x64-5.9.3.run
# Make sure you select the Desktop gcc 64bit option to install in the wizard
# Change your setup to use installed version of qmake
sudo rm /usr/bin/qmake
sudo ln -s $HOME/Qt5.9.3/5.9.3/gcc_64/bin/qmake /usr/bin/qmake
# Build APM Planner 2.0
mkdir ~/workspace
cd ~/workspace/
git clone https://github.com/diydrones/apm_planner
qmake apm_planner.pro
make -j4 # use -j8 on i7
# Run APM Planner 2.0
./release/apmplanner2