write pip delete marker into global build dir upon creation

a "partial" fix for #939.

this change writes the pip delete marker into the global build dir upon creation, so it can be removed during the cleanup step.

this change is *not* helpful if the user already has the global build dir, which will be most pre-existing users.  so, this fix is not so exciting.

#906 (in milestone 1.5), intends to handle this better by moving to mkdtemp build dirs in all cases.