doc: add quickstart to linux and macos

tools/install_dependencies.sh

Index: tools/install_dependencies.sh
diff --git a/tools/install_dependencies.sh b/tools/install_dependencies.sh
index ae98884d1e98f38c281d9718747199b9d1ecfe21..037b8940bd0609f2c5fcff8f4c5c6eeabb8ba087 100755
--- a/tools/install_dependencies.sh
+++ b/tools/install_dependencies.sh
@@ -10,21 +10,35 @@
 
 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