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

.azure-pipelines.yml

@@ -31,6 +31,13 @@ jobs:
         echo "##vso[task.prependpath]$CLANG_DIR"
       displayName: Install clang (OSX)
       condition: and(succeeded(), eq(variables['Agent.OS'], 'Darwin'))
+    - bash: |
+        set -euo pipefail
+        curl -Ls https://github.com/ninja-build/ninja/releases/download/v1.8.2/ninja-mac.zip -o ninja-mac.zip
+        unzip ninja-mac.zip
+        sudo cp -v ninja /usr/local/bin/
+      displayName: Install Ninja (OSX)
+      condition: and(succeeded(), eq(variables['Agent.OS'], 'Darwin'))
     - bash: |
         set -euo pipefail
         curl -f http://releases.llvm.org/8.0.0/clang+llvm-8.0.0-x86_64-linux-gnu-ubuntu-16.04.tar.xz | tar xJf -
@@ -51,5 +58,12 @@ jobs:
         echo "##vso[task.setvariable variable=WASM_NM;]$(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
+    - task: CMake@1
+      inputs:
+        workingDirectory: $(Build.BinariesDirectory)/wasi-libc
+        cmakeArgs: cmake -G Ninja -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_C_COMPILER=clang -DCMAKE_SYSTEM_NAME=wasi -DCMAKE_MODULE_PATH=$(Build.SourcesDirectory)/cmake/Modules $(Build.SourcesDirectory)
+      displayName: Configure
+    - task: CMake@1
+      inputs:
+        cmakeArgs: cmake --build $(Build.BinariesDirectory)/wasi-libc
       displayName: Build