Fix build failures of new APIs on old macOS SDK

New APIs supported from macOS 11.0 cannot build successfully on macOS
10.15, even use @available to control whether a declaration is
available on target platform, but does not work in conditionally
compiling.

Update building.md to require the macOS 11.0 SDK.

Bug: dawn:1054
Change-Id: If22ada959c0c1ef41826f78bd337b912772000b1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/61021
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

docs/building.md

@@ -1,16 +1,28 @@
 # Building Dawn
 
+## System requirements
+
+- Linux
+  - The `pkg-config` command:
+    ```sh
+    # Install pkg-config on Ubuntu
+    sudo apt-get install pkg-config
+    ```
+
+- Mac
+  - [Xcode](https://developer.apple.com/xcode/) 12.2+.
+  - The macOS 11.0 SDK. Run `xcode-select` to check whether you have it.
+    ```sh
+    ls `xcode-select -p`/Platforms/MacOSX.platform/Developer/SDKs
+    ```
+
+## Install `depot_tools`
+
 Dawn uses the Chromium build system and dependency management so you need to [install depot_tools] and add it to the PATH.
 
 [install depot_tools]: http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up
 
-On Linux you need to have the `pkg-config` command:
-```sh
-# Install pkg-config on Ubuntu
-sudo apt-get install pkg-config
-```
-
-Then get the source as follows:
+## Get the code
 
 ```sh
 # Clone the repo as "dawn"
@@ -23,6 +35,8 @@ cp scripts/standalone.gclient .gclient
 gclient sync
 ```
 
+## Build Dawn
+
 Then generate build files using `gn args out/Debug` or `gn args out/Release`.
 A text editor will appear asking build options, the most common option is `is_debug=true/false`; otherwise `gn args out/Release --list` shows all the possible options.