[CMake][Autotools] Enable MPI / parallel features by default.

In the past MPI / parallel features were disabled until the
proper flag was added. This patch changes the behavior to
enable MPI iff it is found.

* Autotools: --disable-parallel and --enable-parallel work as
  before but the documentation is a bit clumsy to minimize changes.
* CMake: USE_MPI gets removed, too. It was not used by the Dune
  core modules. The backwards compatible --disable-parallel remains.

lib/dunecommonam2cmake.lib

@@ -21,12 +21,6 @@ dune_common_options_am2cmake()
 
     default_am2cmake_options $CMAKE_PACKAGES
 
-    # Check for --enable-parallel and activate MPI
-    echo $PARAMS | grep \\-\\-enable-parallel > /dev/null
-    if test "$?" -eq 0 ; then
-	      CMAKE_PARAMS="$CMAKE_PARAMS -DUSE_MPI=ON"
-    fi
-
     # Check for --disable-parallel and deactivate MPI if given
     echo $PARAMS | grep \\-\\-disable-parallel > /dev/null
     if test "$?" -eq 0 ; then

m4/dune_mpi.m4

@@ -99,7 +99,8 @@ AC_DEFUN([DUNE_MPI],[
   # enable/disable parallel features
   AC_ARG_ENABLE(parallel,
     AS_HELP_STRING([--enable-parallel],
-      [Enable the parallel features of Dune. If enabled
+      [Enable the parallel features of Dune, which is enabled by default.
+       Use --disable-parallel to disable parallel features.
        configure will try to determine your MPI automatically. You can
        overwrite this setting by specifying the MPICC variable]))
   AC_SUBST(ENABLE_PARALLEL, "$enable_parallel")
@@ -112,7 +113,7 @@ AC_DEFUN([DUNE_MPI],[
   with_mpi="no"
 
   ## do nothing if --disable-parallel is used
-  AS_IF([test "x$enable_parallel" = "xyes"],[
+  AS_IF([test "x$enable_parallel" != "xno"],[
     ACX_MPI([
       MPICOMP="$MPICC"