Install the devkitARM toolchain of devkitPro as per the instructions on their wiki.
Inside the included MSYS2 environment run:
pacman -S gcc git
To set up the repository:
git clone https://github.com/not-alons/hmfomt
git clone https://github.com/pret/agbcc
cd ./agbcc
./build.sh
./install.sh ../hmfomt
cd ../hmfomt
Place a .gba ROM of Harvest Moon: Friends of Mineral Town (USA) in your hmfomt folder and rename it to "baserom".
To build hmfomt.gba and confirm it matches the official ROM image:
make compare
If an OK is returned, then the installation went smoothly.