build: Run libdmg-hfsplus's DMG tool in make deploy

Previously, the compression of the .iso file to a .dmg file was done
outside of `make deploy' in order to use the faketime-wrapped version of
libdmg-hfsplus's DMG tool.

Specifying the faketime-wrapped version of the DMG tool to ./configure
fixes this and simplifies build scripts.

contrib/gitian-descriptors/gitian-osx.yml

@@ -38,7 +38,7 @@ script: |
 
   WRAP_DIR=$HOME/wrapped
   HOSTS="x86_64-apple-darwin18"
-  CONFIGFLAGS="--enable-reduce-exports --disable-bench --disable-gui-tests GENISOIMAGE=$WRAP_DIR/genisoimage"
+  CONFIGFLAGS="--enable-reduce-exports --disable-bench --disable-gui-tests GENISOIMAGE=$WRAP_DIR/genisoimage DMG=$WRAP_DIR/dmg"
   FAKETIME_HOST_PROGS=""
   FAKETIME_PROGS="ar ranlib date dmg genisoimage"
 
@@ -132,7 +132,6 @@ script: |
 
     make osx_volname
     make deploydir
-    OSX_VOLNAME="$(cat osx_volname)"
     mkdir -p unsigned-app-${i}
     cp osx_volname unsigned-app-${i}/
     cp contrib/macdeploy/detached-sig-apply.sh unsigned-app-${i}
@@ -145,8 +144,7 @@ script: |
     find . | sort | tar --mtime="$REFERENCE_DATETIME" --no-recursion --mode='u+rw,go+r-w,a+X' --owner=0 --group=0 -c -T - | gzip -9n > ${OUTDIR}/${DISTNAME}-osx-unsigned.tar.gz
     popd
 
-    make deploy
-    ${WRAP_DIR}/dmg dmg "${OSX_VOLNAME}.dmg" ${OUTDIR}/${DISTNAME}-osx-unsigned.dmg
+    make deploy OSX_DMG="${OUTDIR}/${DISTNAME}-osx-unsigned.dmg"
 
     cd installed
     find . -name "lib*.la" -delete