Update Makefile/source to be compatible with latest GBDK

[ghost]

While trying to build the source files using the latest version of GBDK (4.1.1 as of writing this), I get the following error:

"at 1: error 131: cannot generate code for target 'gbz80'"

By changing "-mgbz80" to "-msm83 -D__PORT_sm83", I am able to produce the ASM files, but I get the following warnings:

Running compile.sh then produces a ROM file which upon opening, causing the driver to crash/hang. I believe this is an issue with the Makefile and not the engine itself, as I have been able to compile the same MML file using pre-built ASM files that were generated from (what appears to be) another version of GBDK or SDCC. The ROM plays back the music as normal using these pre-built files.
asm-pre-built.zip