Install devkitARM.
Make sure that there is an environment variable called DEVKITARM with the path of the directory before the "bin" directory containing "arm-none-eabi-as", "arm-none-eabi-cpp", "arm-none-eabi-ld" and "arm-none-eabi-objcopy".
Then get the compiler from https://github.com/pret/agbcc and run the following commands.
./build.sh
./install.sh PATH_OF_POKERUBY_DIRECTORY
Then in the pokeruby directory, build the tools.
./build_tools.sh
Finally, build the rom.
make
Install devkitARM.
Then get the compiled tools from https://github.com/pret/pokeruby-tools. Copy the tools/ folder over the tools/ folder in your pokeruby directory.
You can then build pokeruby using make in the MSYS environment provided with devkitARM.
When you simply enter make and don't specify a target, then Pokémon Ruby 1.0 will be built. However, Sapphire can also be built, along with revisions 1 and 2 of both Ruby and Sapphire. Here is a listing of each ROM that can be made, along with the command to make the ROM.
Pokémon Ruby 1.0
make ruby
Pokémon Ruby 1.1
make ruby_rev1
Pokémon Ruby 1.2
make ruby_rev2
Pokémon Sapphire 1.0
make sapphire
Pokémon Sapphire 1.1
make sapphire_rev1
Pokémon Sapphire 1.2
make sapphire_rev2