Minor Build Script Updates, Mac OS Support

* Update the Makefile to choose MacOS friendly `date` formats.
* Build the Windows doodle.exe binary as a GUI application to skip the
  console window.
* Added Mac OS build instructions.

dev-assets/doodads/build.sh

@@ -98,7 +98,6 @@ azulians() {
 
 	doodad convert -t "Blue Azulian" blu-front.png blu-back.png \
 		blu-wr{1,2,3,4}.png blu-wl{1,2,3,4}.png azu-blu.doodad
-	doodad edit-doodad -hide azu-blu.doodad
 	doodad install-script azulian.js azu-blu.doodad
 
 	doodad convert -t "Red Azulian" red-front.png red-back.png \
@@ -128,3 +127,4 @@ trapdoors
 azulians
 objects
 doodad edit-doodad -quiet -lock -author "Noah" ../../assets/doodads/*.doodad
+doodad edit-doodad -hide ../../assets/doodads/azu-blu.doodad