use proper quoting to avoid autoconf warning on missing AC_LANG_SOURCE

[[Imported from SVN: r6497]]

m4/dune_mpi.m4

@@ -149,11 +149,11 @@ AC_DEFUN([DUNE_MPI],[
     # try to create MPI program
     AC_LANG_PUSH([C++])
     AC_COMPILE_IFELSE(
-      AC_LANG_SOURCE(
+      [AC_LANG_SOURCE(
         [ #include <mpi.h>
           int main (int argc, char** argv) { 
           MPI_Init(&argc, &argv); 
-          MPI_Finalize(); }]),
+          MPI_Finalize(); }])],
         [ AC_MSG_RESULT([yes]) ],
         [ AC_MSG_RESULT([no])
           AC_MSG_ERROR([could not compile MPI testprogram!
@@ -166,11 +166,11 @@ AC_DEFUN([DUNE_MPI],[
     ],[
       AC_MSG_CHECKING([whether running with $dune_MPI_VERSION works])
       AC_RUN_IFELSE(
-        AC_LANG_SOURCE(
+        [AC_LANG_SOURCE(
           [ #include <mpi.h>
             int main (int argc, char** argv) { 
             MPI_Init(&argc, &argv); 
-            MPI_Finalize(); }]),
+            MPI_Finalize(); }])],
           [ AC_MSG_RESULT([yes]) ],
           [ AC_MSG_RESULT([no])
             AC_MSG_ERROR([could not run MPI testprogram!