[minor] p4est-setup.sh can now also build from source directory

Before p4est-setup.sh could only build from a tar.gz. Now you can also specify a source directory instead. To build p4est from source:
  $ git clone https://github.com/cburstedde/p4est.git .
  $ git submodule update --init; ./bootstrap
  $ /dealii/doc/external-libs/p4est-setup.sh `pwd` `pwd`/install