Generate GMM headers for pl_msg at build-time

[lhearachel]

No description provided.

[mid-kid]

This change expects that the build dir is always directly one level under the source directory. I think there's ways to get a full path to the source directory in meson, but putting build artifacts in the source directory is really bad practice.

Please explore the possibility for generating these files in the build directory, and adding the include path to the compiler include path (through its command line) instead. See https://mesonbuild.com/Include-directories.html#include-directories

[lhearachel]

Please explore the possibility for generating these files in the build directory, and adding the include path to the compiler include path (through its command line) instead.

I got this working, with the added caveat that it requires res to be walked before src, but that's not a huge deal, IMO.

[mid-kid]

Thanks! It looks nice now.

[lhearachel]

Tested this out a bit locally with some simple modifications to use the generated headers in battle_script.c, and it works as expected!