> If you use install -D below I don't think you need this line. 

It is needed, otherwise will report xxx/pnacl is not a directory.  

> Additionally, can't you just use install -p -D in the line below like we do in the others?

Yes, I can.  But files under pnacl varies on different  arch. if I list them one by one, there will be lots of %ifarch. That will make the spec much longer, so I do not like that.