GN: quick doc

site/user/quick/gn.md

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+GN
+=====
+
+[GN](https://chromium.googlesource.com/chromium/src/tools/gn/)
+is a new meta-build system originally designed to replace GYP in Chromium.
+
+You can build Skia using GN in a limited number of configurations.  We expect
+that as that limited number rises, GN will become the preferred, and then only,
+way to build Skia.
+
+Supported Features
+----------
+
+    * Linux, Mac
+    * Software rendering
+    * libskia.a, libskia.so
+    * DM, nanobench
+
+Quickstart
+----------
+
+Please check out Skia using the instructions in one of the other quick start
+guides.  We diverge where they'd first run some command with "gyp" in it.
+
+<!--?prettify lang=sh?-->
+
+    # After gclient sync, run fetch-gn to make sure you have GN.
+    gclient sync && bin/fetch-gn

[hal.canary, 2016/08/03 18:26:45]

`bin/sync` is a good wrapper around `gclient sync`

+
+    # Run GN to generate your build files.  Some examples.
+    gn gen out/Release
+    gn gen out/Debug --args=is_debug=true
+    gn gen out/Clang --args='cc="clang" cxx="clang++"'
+    gn gen out/Shared --args=is_component_build=true
+
+    # Build
+    ninja -C out/Release
+    ninja -C out/Debug
+    ninja -C out/Clang
+    ninja -C out/Shared
+
+From here everything is pretty much business as usual.