Always use Ninja generator when building cmake projects form source

.circleci/config.yml

@@ -29,7 +29,7 @@ commands:
       - checkout
       - run:
           name: Install CMake
-          command: brew install cmake
+          command: brew install cmake ninja
   test-macos:
     steps:
       - run:
@@ -132,7 +132,7 @@ jobs:
       - checkout
       - run:
           name: Install debian packages
-          command: apt-get update -q && apt-get install -q -y cmake build-essential openjdk-8-jre-headless ksh zsh
+          command: apt-get update -q && apt-get install -q -y cmake ninja-build build-essential openjdk-8-jre-headless ksh zsh
       - run: test/test_node_path.sh
       - run: test/test.sh
       - run: test/test_source_env.sh
@@ -147,7 +147,7 @@ jobs:
       - checkout
       - run:
           name: Install debian packages
-          command: sudo apt-get update -q && sudo apt-get install -q cmake build-essential openjdk-8-jre-headless
+          command: sudo apt-get update -q && sudo apt-get install -q cmake ninja-build build-essential openjdk-8-jre-headless
       - run: test/test.sh
   test-mac-arm64:
     executor: mac_arm64

.gitignore

@@ -30,7 +30,6 @@ __pycache__
 /downloads
 /crunch
 /java
-/mingw
 /spidermonkey
 /binaryen
 /firefox

README.md

@@ -50,7 +50,7 @@ important concepts to help understanding the internals of the SDK:
 
 Using the emsdk pre-compiled packages requires only the minimal set of
 dependencies lists below.  When building from source a wider set of tools
-include git, cmake, and a host compiler are required. See:
+including git, cmake, ninja, and a host compiler are required. See:
 https://emscripten.org/docs/building_from_source/toolchain_what_is_needed.html.
 
 ### Mac OS X
@@ -78,6 +78,9 @@ so there should be no issues with older versions of `libstdc++` or `libc++`.
 
 - `java`: For running closure compiler (optional)
 
+If building from source, you must run `emsdk` inside a Visual Studio Developer
+Command Prompt (`cl.exe` in `PATH`) and have `ninja` installed.
+
 ## Uninstalling the Emscripten SDK
 
 To remove the Emscripten SDK, simply delete the emsdk directory.
@@ -143,6 +146,13 @@ emsdk install sdk-main-64bit
 emsdk activate sdk-main-64bit
 ```
 
+Note that when building from source (`emsdk install sdk-main-64bit`,
+`binaryen-main-64bit`, or `llvm-git-main-64bit`), `emsdk` uses the Ninja
+generator (`-G Ninja`). You must have `ninja` installed in your `PATH`. On
+Windows, you must also invoke `emsdk` from a Visual Studio Developer Command
+Prompt (where `cl.exe` and Visual Studio environment variables are loaded in
+your shell).
+
 You can use this target for example to bootstrap developing patches to LLVM,
 Binaryen or Emscripten. (After initial installation, use `git remote add`
 in the cloned tree to add your own fork to push changes as patches)