cmake_build: support SKIA_OUT and BUILDTYPE

cmake/cmake_build

 #!/bin/sh
 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Script for building with CMake on Skia build bots.
 # If you have CMake installed on your system, you can just use it normally.
 
 set -e
 set -x
 
-here=`dirname $0`
+fulldir() {
+    cd `dirname $1`
+    echo `pwd`
+}
+
+here=`fulldir $0`
 cores=32
 
+if [[ -z "$BUILDTYPE" || -z "$SKIA_OUT" ]]; then
+    echo "If you're a bot, you should be setting BUILDTYPE ($BUILDTYPE) and SKIA_OUT ($SKIA_OUT)."
+    echo "If you're not a bot, you don't want to run this script."
+    exit 1
+fi
+
 echo "Bootstrapping CMake"
 pushd $here/../third_party/externals/cmake
 ./bootstrap --parallel=$cores
 make -j $cores
 popd
 
 echo "Building with bootstrapped CMake"
-pushd $here
-../third_party/externals/cmake/bin/cmake . -G Ninja
+mkdir -p $SKIA_OUT
+pushd $SKIA_OUT
+$here/../third_party/externals/cmake/bin/cmake -DCMAKE_BUILD_TYPE=$BUILDTYPE $here -G Ninja
 ninja skia  # The default also builds `example`, which I'm leaving a TODO for now.
 popd