Support SerenityOS

[matyalatte]

SerenityOS is a modern Unix-like system with retro UI. A test case (envuSetEnvEmpy) fails but I think it's SerenityOS's issue.

TEST(UtilTest, envuSetEnvEmpty) {
    // This should do nothing.
    int ret = envuSetEnv("", "test");
    EXPECT_EQ(-1, ret);
    char* env = envuGetEnv("");
    ASSERT_EQ(NULL, env);
}

Commands I executed

# on Ubuntu 24.04
sudo apt update
# Required for OS
sudo apt install git build-essential cmake curl libmpfr-dev libmpc-dev libgmp-dev e2fsprogs ninja-build qemu-system-gui qemu-system-x86 qemu-utils ccache rsync unzip texinfo libssl-dev
# Required for Ports
sudo apt install imagemagick zlib1g-dev m4 gettext

# Clone repo
git clone https://github.com/SerenityOS/serenity.git
cd serenity

# Build OS and toolchain
./Meta/serenity.sh build
./Toolchain/BuildPython.sh

# Include ports
# Note: You can't do "./Ports/*/package.sh"
# Note: You should build gcc before building python3
for dir in bash curl git pkgconf gcc ninja python3; do
    (cd "Ports/$dir" && ./package.sh)
done

# Copy certificate files from host machine. (python's ssl module uses them.)
cp /usr/lib/ssl/certs/* Build/x86_64/Root/usr/local/ssl/certs/

# Build and run Serenity OS
Meta/serenity.sh run

# -------------------------
# on Serenity OS
cd ~/
git clone --depth 1 https://github.com/mesonbuild/meson.git
git clone https://github.com/matyalatte/c-env-utils.git
cd c-env-utils

# Build and test
python3 ~/meson/meson.py setup build
python3 ~/meson/meson.py compile -C build
python3 ~/meson/meson.py test -C build
./build/env_utils_cli