LDFLAGS-transition for the MPI stuff:

  - Introduce autoconf substitutions DUNEMPICPPFLAGS, DUNEMPILDFLAGS and
    DUNEMPILIBS.
  - Deprecate MPI_LDFLAGS and MPI_CPPFLAGS: the automake manual recommends
    against "_" in names of such variables, MPI_LDFLAGS actually contains
    mainly libraries and simply changing MPI_LDFLAGS to MPI_LIBS will probably
    break a lot of stuff.  By deprecating MPI_CPPFLAGS it is possible to make
    the preprocessor generate a deprecation warning with instructions to
    change both MPI_CPPFLAGS and MPI_LDFLAGS (though this has yet to be
    implemented).

[[Imported from SVN: r5806]]