doc: add quickstart to linux and macos

tools/install_dependencies.sh

 #!/bin/sh
 # Copyright 2014 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # install_dependencies.sh will install system-specific Skia
 # dependencies using your system's package manager.  If your system is
 # not supported, add logic here to support it.
 
 set -e
 
+# Return 0 iff all package name arguments are installed.
+dpkg_all_installed() {
+    for arg; do
+        if !(dpkg-query -W -f'${Status}' "$arg" 2>/dev/null | \
+            grep -q "ok installed"); then
+            return 1
+        fi
+    done
+    return 0
+}
+
 if command -v lsb_release > /dev/null ; then
     case $(lsb_release -i -s) in
         Ubuntu)
-            sudo apt-get install \
-                build-essential \
-                libfreetype6-dev \
-                libfontconfig-dev \
-                libpng12-dev \
-                libgif-dev \
-                libqt4-dev \
-                clang
-            if [ $(lsb_release -r -s) = '14.04' ] ; then
-                sudo apt-get install \
-                    ninja-build
-            fi
+            PACKAGES=$(cat<<-EOF
+                build-essential
+                libfreetype6-dev
+                libfontconfig-dev
+                libpng12-dev
+                libgif-dev
+                libqt4-dev
+                EOF
+            )
+           if [ $(lsb_release -r -s) = '14.04' ] ; then
+               PACKAGES="${PACKAGES} ninja-build"
+            fi
+            if ! dpkg_all_installed $PACKAGES; then
+                sudo apt-get install $PACKAGES
+            fi
             exit
             ;;
     esac
 fi
 
 echo 'unknown system'
 exit 1