embree is now a submodule

Former-commit-id: de880eee2779c9f58de7151ed75cd575ec7a7f0f

.gitmodules

@@ -4,3 +4,6 @@ url=https://github.com/libigl/nanogui.git
         fetchRecursiveSubmodules = true
         ignore = dirty
 
+[submodule "external/embree"]
+	path = external/embree
+	url = https://github.com/embree/embree.git