tce-load -i compiletc qt-5.x-bin qt-5.x-dev muparser-dev boost-1.65-dev

cd LibreCAD-2.2.0-rc1

Edit /usr/local/lib/qt5/mkspecs/common/g++-base.conf

QMAKE_CXXFLAGS_GNUCXX11 = -D_GLIBCXX_USE_C99_MATH -std=gnu++11

qmake -r

make [14m 53.71s]
sudo make install

git clone https://github.com/LibreCAD/LibreCAD
https://github.com/LibreCAD/LibreCAD/wiki/Build-from-source