Automated builds for SDL and plugins via GitHub actions


I am trying to generate automated builds for SDL and the plugins with GitHub actions. I am using the unofficial GitHub mirrors of the Hg repositories, which can be found here: What I need is to produce dynamic libraries (*.dll for Windows, *.dylib for MacOS and *.so for Linux - Ubuntu) to be used by a C# application via platform invoke (PInvoke). I have created a GitHub action for the SDL repository using CMake and everything compiles just fine on all platforms. What gives me trouble are the plugins. I need builds for SDL Image, SDL Mixer and SDL TTF, at least. Apparently the plugins are not setup to be built via CMake. I have tried the regular Make, but the build fails giving me an error it cannot discover the SDL headers and binaries. What do I need to configure on the build agents for the build to succeed? I am sorry to ask such a silly question, but I am a C# guy and all this C++ build stuff is new to me. Any help would be greatly appreciated.