Fix MPI/OpenMP-related compile issues

src/initialize.F90

@@ -23,7 +23,9 @@ module initialize
   use output,          only: print_version, write_message, print_usage
   use random_lcg,      only: initialize_prng
   use settings
+#ifdef _OPENMP
   use simulation_header, only: n_threads
+#endif
   use string,          only: to_str, starts_with, ends_with, str_to_int
   use tally_header,    only: TallyObject
   use tally_filter

src/simulation.F90

@@ -17,6 +17,7 @@ module simulation
   use eigenvalue,      only: join_bank_from_threads
 #endif
   use error,           only: fatal_error
+  use geometry_header, only: n_cells
   use message_passing
   use mgxs_header,     only: energy_bins, energy_bin_avg
   use nuclide_header,  only: micro_xs, n_nuclides

src/source_header.F90

@@ -216,7 +216,6 @@ contains
     class(SourceDistribution), intent(in) :: this
     type(Bank) :: site
 
-    real(8) :: r(3)       ! sampled coordinates
     logical :: found      ! Does the source particle exist within geometry?
     type(Particle) :: p   ! Temporary particle for using find_cell
 
@@ -305,7 +304,6 @@ contains
     integer(C_INT32_T), optional, intent(out) :: index_end
     integer(C_INT) :: err
 
-    integer :: i
     type(SourceDistribution), allocatable :: temp(:) ! temporary array
 
     if (n_sources == 0) then