MPITOOLS: Fix compilation without MPI.

Signed-off-by: Grzegorz Kowal <grzegorz@amuncode.org>

sources/mpitools.F90

@@ -44,6 +44,7 @@ module mpitools
 
 ! subroutine interfaces
 !
+#ifdef MPI
   interface reduce_minimum
     module procedure reduce_minimum_double_array
   end interface
@@ -57,6 +58,7 @@ module mpitools
     module procedure reduce_sum_double_array
     module procedure reduce_sum_complex_array
   end interface
+#endif /* MPI */
 
 ! timer indices
 !
@@ -82,9 +84,11 @@ module mpitools
 !
   public :: initialize_mpitools, finalize_mpitools
   public :: check_status
+#ifdef MPI
   public :: reduce_minimum, reduce_maximum, reduce_sum
   public :: send_array, receive_array
   public :: exchange_arrays
+#endif /* MPI */
 
 ! declare public variables
 !