Warnings while compiling to WebAssembly

Hi! I am working on compiling ROS2 nodes to WebAssembly using Emscripten so that I can simulate robot control software in the browser. For the most part, this is going smoothly so far but there are a couple warnings that I think should be addressed:

1. Conditional compilation in rcutils_get_platform_library_name

rcutils/src/shared_library.c:289:16: warning: unused parameter 'buffer_size' [-Wunused-parameter]
rcutils/src/shared_library.c:290:8: warning: unused parameter 'debug' [-Wunused-parameter]

It seems when I compile using Clang/Emscripten, none of the #ifdefs are matching my configuration in rcutils_get_platform_library_name, which is a bit unusual since I was under the impression that Emscripten defines __LINUX__ (uppercase). I will investigate this, however, there is also the possibility of checking whether __EMSCRIPTEN__ has been defined.

2. Conditional compilation in rcutils_strerror

rcutils/src/strerror.c:32:10: warning: incompatible integer to pointer conversion initializing 'char *' with an expression of type 'int' [-Wint-conversion]

This warning seems very similar to the previous one. It seems again that conditional compilation has gone down the wrong path since the version strerror_r that comes with Emscripten's C standard library returns an int instead of a char*.

For reference, I am using the Emscripten 3.1.0 (Clang 14.0.0) with the following colcon mixin:

{
    "build": {
        "emscripten": {
            "cmake-args": [
                "-DCMAKE_TOOLCHAIN_FILE=/path/to/emsdk/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake",
                "-DCMAKE_STAGING_PREFIX=/path/to/developer/emsdk/upstream/emscripten/cache/sysroot/usr/",
                "-DBUILD_SHARED_LIBS=OFF",
                "-DBUILD_TESTING=OFF",
                "--no-warn-unused-cli"
            ]
        }
    }
}

Start with src/shared_library.c at rcutils_get_platform_library_name and src/strerror.c at rcutils_strerror, using the reported Emscripten toolchain and warnings as the initial checks. Inspect the platform conditionals and Emscripten's strerror_r behavior; done means the relevant WebAssembly compilation path no longer emits these warnings.