Use the same Makefile build system as Gen 1+2 repos for macOS compatibility

[Rangi42]

macOS comes with BSD make, which is more limited than GNU make. The system that pokegold-spaceworld uses for putting object files in a build/ directory breaks the build on macOS:

% make compare
make: *** No rule to make target `build/home/audio.o', needed by `pokegold-spaceworld.gb'.  Stop.

Users have to install GNU make from Homebrew or MacPorts:

% gmake compare
mkdir -p build/home/
mkdir -p build/engine/battle/
...
rgbasm -o rgbdscheck.o rgbdscheck.asm
rgbasm -E -i build/ -DGOLD -o build/home/audio.o home/audio.asm
rgbasm -E -i build/ -DGOLD -o build/home/bankswitch.o home/bankswitch.asm
...
pokegold-spaceworld.gb: OK
pokegold-spaceworld-correctheader.gb: OK