following advince from FFTW3 manual

silences a lot of gfortran warnings about unused variables

src/grid/FFTW.f90

@@ -0,0 +1,13 @@
+!--------------------------------------------------------------------------------------------------
+!> @author Martin Diehl, KU Leuven
+!> @brief Wrap FFTW3 into a module.
+!--------------------------------------------------------------------------------------------------
+module FFTW3
+  use, intrinsic :: ISO_C_binding
+
+  implicit none
+  public
+
+  include 'fftw3-mpi.f03'
+
+end module FFTW3

src/grid/discretization_grid.f90

@@ -10,6 +10,7 @@ module discretization_grid
 #if (PETSC_VERSION_MAJOR==3 && PETSC_VERSION_MINOR>14) && !defined(PETSC_HAVE_MPI_F90MODULE_VISIBILITY)
   use MPI_f08
 #endif
+  use FFTW3
 
   use prec
   use parallelization
@@ -50,7 +51,6 @@ subroutine discretization_grid_init(restart)
 
   logical, intent(in) :: restart
 
-  include 'fftw3-mpi.f03'
   real(pReal), dimension(3) :: &
     mySize, &                                                                                       !< domain size of this process
     origin                                                                                          !< (global) distance to origin

src/grid/spectral_utilities.f90

@@ -4,13 +4,12 @@
 !> @brief Utilities used by the different spectral solver variants
 !--------------------------------------------------------------------------------------------------
 module spectral_utilities
-  use, intrinsic :: iso_c_binding
-
 #include <petsc/finclude/petscsys.h>
   use PETScSys
 #if (PETSC_VERSION_MAJOR==3 && PETSC_VERSION_MINOR>14) && !defined(PETSC_HAVE_MPI_F90MODULE_VISIBILITY)
   use MPI_f08
 #endif
+  use FFTW3
 
   use prec
   use CLI
@@ -26,8 +25,6 @@ module spectral_utilities
   implicit none
   private
 
-  include 'fftw3-mpi.f03'
-
 !--------------------------------------------------------------------------------------------------
 ! grid related information
   real(pReal), protected,  public                :: wgt                                             !< weighting factor 1/Nelems