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 compiled tools from https://github.com/YamaArashi/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.