add support for Debian 9 to preinstall.sh

preinstall.sh

@@ -125,8 +125,8 @@ if [ $enable_openslide -eq 1 ] && [ -z $vips_with_openslide ] && [ $openslide_ex
     DISTRO=$(lsb_release -c -s)
     echo "Detected Debian Linux '$DISTRO'"
     case "$DISTRO" in
-      jessie|vivid|wily|xenial)
+      jessie|vivid|wily|xenial|stretch)
-        # Debian 8, Ubuntu 15
+        # Debian 9, Debian 8, Ubuntu 15
         echo "Installing libopenslide via apt-get"
         apt-get install -y libopenslide-dev
         ;;
@@ -215,6 +215,12 @@ if [ -f /etc/debian_version ]; then
       apt-get install -y automake build-essential gobject-introspection gtk-doc-tools libglib2.0-dev libjpeg-dev libpng12-dev libwebp-dev libtiff5-dev libexif-dev libgsf-1-dev liblcms2-dev libxml2-dev swig libmagickcore-dev curl
       install_libvips_from_source
       ;;
+    stretch)
+      # Debian 9
+      echo "Installing libvips dependencies via apt-get"
+      apt-get install -y automake build-essential gobject-introspection gtk-doc-tools libglib2.0-dev libjpeg-dev libpng-dev libwebp-dev libtiff5-dev libexif-dev libgsf-1-dev liblcms2-dev libxml2-dev swig libmagickcore-dev curl
+      install_libvips_from_source
+      ;;
     precise|wheezy|maya)
       # Debian 7, Ubuntu 12.04, Mint 13
       echo "Installing libvips dependencies via apt-get"