You need to install the build and runtime dependancies:
pkg install mysql80-server
pkg install cmake
pkg install boost-all
Clone the project
git clone https://github.com/azerothcore/azerothcore-wotlk.git --branch master --single-branch azerothcore
Enter the project's directory
cd azerothcore
Create the build directory
mkdir build
Enter the directory
cd build
Configure azerothcore for building
cmake ../ -DCMAKE_INSTALL_PREFIX=$HOME/azeroth-server/ -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_COMPILER=/usr/bin/clang++ -DWITH_WARNINGS=1 -DTOOLS=0 -DSCRIPTS=static
Compile Azerothcore
make -j<number of cores>
Install Azerothcore
make install