configure: remove usage of sudo with external-packages

and print appropriate error message when installing an external-package in read-only prefix location use 'PrefixWriteCheck' in petsc4py to prevent this check [as its not really an external-package]

balay@sb /home/balay/petsc (balay/configure-remove-sudo=)
$ ./configure --prefix=/usr/local/petsc --download-metis --with-fortran-bindings=0
=============================================================================================
                      Configuring PETSc to compile on your system                            
=============================================================================================
TESTING: configureLibrary from config.packages.metis(config/BuildSystem/config/packages/metis.py:46)                                                                                                               *******************************************************************************
         UNABLE to CONFIGURE with GIVEN OPTIONS    (see configure.log for details):
-------------------------------------------------------------------------------
Specified prefix-dir: /usr/local/petsc is read-only! "--download-metis" cannot install at this location! Suggest:
      sudo mkdir /usr/local/petsc
      sudo chown $USER /usr/local/petsc
Now rerun configure
*******************************************************************************