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
ninja -C build install
```

Author: compnerd

.azure-pipelines.yml

@@ -33,10 +33,8 @@ jobs:
       condition: and(succeeded(), eq(variables['Agent.OS'], 'Darwin'))
     - bash: |
         set -euo pipefail
-        curl -f http://releases.llvm.org/8.0.0/clang+llvm-8.0.0-x86_64-linux-gnu-ubuntu-16.04.tar.xz | tar xJf -
-        export CLANG_DIR=`pwd`/clang+llvm-8.0.0-x86_64-linux-gnu-ubuntu-16.04/bin
-        echo "##vso[task.prependpath]$CLANG_DIR"
-      displayName: Install clang (Linux)
+        sudo apt-get -y install ninja-build
+      displayName: Install Ninja (Linux)
       condition: and(succeeded(), eq(variables['Agent.OS'], 'Linux'))
     - bash: |
         # Windows releases of LLVM don't include the llvm-nm tool, which is needed for building
@@ -51,5 +49,9 @@ jobs:
         echo "##vso[task.setvariable variable=WASM_NM;]$(rustc --print sysroot|sed 's|C:|/c|'|sed 's|\\|/|g')/lib/rustlib/x86_64-pc-windows-msvc/bin/llvm-nm.exe"
       displayName: Install llvm-nm (Windows)
       condition: and(succeeded(), eq( variables['Agent.OS'], 'Windows_NT' ))
-    - script: make -j4
-      displayName: Build
+    - script: |
+        cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_C_COMPILER=clang -S $(Build.SourcesDirectory) -B $(Build.BinariesDirectory)
+        cmake --build $(Build.BinariesDirectory)
+    - publish: $(Build.BinariesDirectory)/sysroot
+      artifact: wasi-libc
+      condition: and(succeeded(), eq(variables['Agent.OS'], 'Darwin'))