build: add cmake based build infrastructure

Rather than rely on make, use CMake to generate the rules in the build
system of choice.  This allows building with `make` or `ninja`, and
enables building the WASI libc on Windows.

```
cmake -G Ninja -B build -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_C_COMPILER=clang -DCMAKE_SYSTEM_NAME=wasi -DCMAKE_MODULE_PATH=cmake/Modules
ninja -C build install
```

Author: compnerd