Toolchain: Add a script to build GN from source

Toolchain/BuildGN.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+
+# This script builds the GN meta-build system
+set -e
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+# shellcheck source=/dev/null
+. "${DIR}/../Meta/shell_include.sh"
+
+exit_if_running_as_root "Do not run BuildGN.sh as root, parts of your Toolchain directory will become root-owned"
+
+NPROC=$(get_number_of_processing_units)
+[ -z "$MAKEJOBS" ] && MAKEJOBS=${NPROC}
+
+GIT_REPO=https://gn.googlesource.com/gn
+GIT_REV=fae280eabe5d31accc53100137459ece19a7a295
+BUILD_DIR="$DIR"/Build/gn
+PREFIX_DIR="$DIR/Local/gn"
+
+mkdir -p "$DIR"/Tarballs
+pushd "$DIR"/Tarballs
+
+[ ! -d gn ] && git clone $GIT_REPO
+
+cd gn
+git fetch origin
+git checkout $GIT_REV
+
+./build/gen.py --out-path="$BUILD_DIR" --allow-warnings
+mkdir -p "$PREFIX_DIR/bin"
+cp "$BUILD_DIR/gn" "$PREFIX_DIR/bin"
+
+popd