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=Generic
ninja -C build install
```

Author: compnerd

.azure-pipelines.yml

@@ -51,5 +51,24 @@ jobs:
         echo "##vso[task.setvariable variable=WASM_NM;]$(rustc --print sysroot|sed 's|C:|/c|'|sed 's|\\|/|g')/lib/rustlib/x86_64-pc-windows-msvc/bin/llvm-nm.exe"
       displayName: Install llvm-nm (Windows)
       condition: and(succeeded(), eq( variables['Agent.OS'], 'Windows_NT' ))
-    - script: make -j4
+    - bash: |
+        echo "##vso[task.setvariable variable=LLVM_AR]$(which llvm-ar)"
+      displayName: Find llvm-ar (!Windows)
+      condition: and(succeeded(), not(eq(variables['Agent.OS'], 'Windows_NT')))
+    - script: |
+        echo ##vso[task.setvariable variable=LLVM_AR]%CD%\citools\clang-rust\bin\llvm-ar.exe
+      displayName: Find llvm-ar (Windows)
+      condition: and(succeeded(), eq(variables['Agent.OS'], 'Windows_NT'))
+    - task: CMake@1
+      inputs:
+        workingDirectory: $(Build.BinariesDirectory)
+        cmakeArgs: -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_SYSTEM_NAME=Generic -DCMAKE_AR=$(LLVM_AR) -DCMAKE_C_COMPILER=clang -S $(Build.SourcesDirectory)
+      displayName: Configure
+    - task: CMake@1
+      inputs:
+        workingDirectory: $(Build.BinariesDirectory)
+        cmakeArgs: --build .
       displayName: Build
+    - publish: $(Build.BinariesDirectory)/sysroot
+      artifact: wasi-libc
+      condition: and(succeeded(), eq(variables['Agent.OS'], 'Darwin'))