Make only binary distributions use laid-out DMGs.

This is done by adding an "ldmg" type for laid-out DMGs, and "dmg"
produces a plain container image as before.
(cherry picked from commit a2207e6)

collects/meta/build/build

@@ -322,15 +322,15 @@ platforms_of_dist_type() {
 }
 installer_of_dist_type_platform() { # input: dtype-dplatform
   case "$1" in
-    ( "src-unix"        ) echo "tgz" ;;
-    ( "src-mac"         ) echo "dmg" ;;
-    ( "src-win"         ) echo "zip" ;;
-    ( "bin-"*"-linux"*  ) echo "sh"  ;;
-    ( "bin-"*"-freebsd" ) echo "sh"  ;;
-    ( "bin-"*"-solaris" ) echo "sh"  ;;
-    ( "bin-"*"-darwin"  ) echo "sh"  ;;
-    ( "bin-"*"-osx-mac" ) echo "dmg" ;;
-    ( "bin-"*"-win32"   ) echo "exe" ;;
+    ( "src-unix"        ) echo "tgz"  ;;
+    ( "src-mac"         ) echo "dmg"  ;;
+    ( "src-win"         ) echo "zip"  ;;
+    ( "bin-"*"-linux"*  ) echo "sh"   ;;
+    ( "bin-"*"-freebsd" ) echo "sh"   ;;
+    ( "bin-"*"-solaris" ) echo "sh"   ;;
+    ( "bin-"*"-darwin"  ) echo "sh"   ;;
+    ( "bin-"*"-osx-mac" ) echo "ldmg" ;;
+    ( "bin-"*"-win32"   ) echo "exe"  ;;
     ( * ) exit_error "Unknown dist type+platform for" \
                      "installer_of_dist_type_platform: \"$1\"" ;;
   esac
@@ -341,6 +341,8 @@ explanation_of_installer_type() {
                     "\"gunzip <file> | tar xvf -\"." ;;
     ( "dmg"  ) echo "Mount this disk image and copy the Racket folder to" \
                     "your disk." ;;
+    ( "ldmg" ) echo "Mount this disk image and copy the Racket folder to" \
+                    "the Applications folder on your disk." ;;
     ( "zip"  ) echo "Use unzip to extract the Racket folder to your disk." ;;
     ( "sh"   ) echo "Execute this file with \"sh <file>\"," \
                     "and follow the instructions." ;;
@@ -1721,7 +1723,7 @@ make_dmg() { # inputs: dir, dmg, bg-image
   local src="$(basename "$srcdir")"
   local myself="$(id -nu):$(id -ng)"
   show "Making \"$tgtdmg\" from \"$srcdir\""
-  _cp "$tmpbg" "$srcdir"
+  if [[ "x$tmpbg" != "x-" ]]; then _cp "$tmpbg" "$srcdir"; fi
   _cd "$(dirname "$srcdir")"
   _run sudo rm -f "$tgtdmg" "$tmpdmg"
   # It should be possible to create dmgs normally, but they'd be created with
@@ -1739,7 +1741,9 @@ make_dmg() { # inputs: dir, dmg, bg-image
   # [2] remove the source tree
   _run sudo rm -rf "$src"
   # [3] do the expected dmg layout (see below)
-  easy_dmg_layout "$tmpdmg" "$src" "$(basename "$tmpbg")"
+  if [[ "x$tmpbg" != "x-" ]]; then
+    easy_dmg_layout "$tmpdmg" "$src" "$(basename "$tmpbg")"
+  fi
   # [4] create the compressed image from the uncompressed image
   _run sudo hdiutil convert -format UDZO -imagekey zlib-level=9 -ov \
                        "$tmpdmg" -o "$tgtdmg"
@@ -1805,18 +1809,27 @@ do_tgz_to_dmg() {
   _cd "$savedpwd"
   _rm "$tmpdir/tgz-to-dmg-$$"
 }
+do_layout_bg="-"
 tgz_to_dmg() {
   local srctgz="$1" tgt="$2" pname="$3" ptype="$4" srcplatform="$5"; shift 5
   local tmptgz="$tmpdir/tgz2dmg.tgz"
   local tmpdmg="$tmpdir/tgz2dmg.dmg"
-  local tmpbg="$tmpdir/bg.png"
+  local tmpbg="-"
   _scp "$srctgz" "${dmgmachine}:$tmptgz"
-  _scp "$PLTHOME/$dmgbackground" "${dmgmachine}:$tmpbg"
+  if [[ "x$do_layout_bg" != "x-" ]]; then
+    _scp "$do_layout_bg" "${dmgmachine}:$tmpbg"
+    tmpbg="$tmpdir/bg.png"
+  fi
   run_part "$dmgmachine" "do_tgz_to_dmg" \
              "$tmptgz" "$tmpdmg" "$tmpbg" \
              "$version" "$pname" "$ptype" "$srcplatform"
   _scp "${dmgmachine}:$tmpdmg" "$tgt.dmg"
 }
+tgz_to_ldmg() {
+  do_layout_bg="$PLTHOME/$dmgbackground"
+  tgz_to_dmg "$@"
+  do_layout_bg="-"
+}
 #----------------------------------------
 do_tgz_to_exe() {
   local tmptgz="$1" tmpexe="$2" nsistgz="$3"