Run additional check for the external32 packing at the configure stage.

config/checkdoc/test_mpi_pack_external_4.txt

+test_mpi_pack_external_1.txt
\ No newline at end of file

config/checksrc/test_mpi_pack_external_4.c

+/* This test checks whether the MPI library supports external32 packing
+   of the types transfered by YAC (see definition of get_mpi_datatype
+   in ../../src/communicator_mpi.c) */
+
+#include <stdint.h>
+#include <limits.h>
+#include <mpi.h>
+#include <stdio.h>
+
+char buf[100]; // should be enough to keep all the packed variables
+MPI_Aint position = 0;
+#define PACK_EXTERNAL(mpi_type) printf("Packing " #mpi_type "\n"); MPI_Pack_external("external32", &mpi_type ## _var, 1, mpi_type, buf, sizeof(buf), &position);
+
+double MPI_DOUBLE_var;
+unsigned int MPI_UNSIGNED_var;
+signed int MPI_INT_var;
+unsigned char MPI_BYTE_var;
+char MPI_CHAR_var;
+char MPI_PACKED_var;
+
+#if SIZE_MAX == UCHAR_MAX
+unsigned char SIZE_T_var;
+# define SIZE_T MPI_UNSIGNED_CHAR
+#elif SIZE_MAX == USHRT_MAX
+unsigned short SIZE_var;
+# define SIZE_T MPI_UNSIGNED_SHORT
+#elif SIZE_MAX == UINT_MAX
+unsigned int SIZE_T_var;
+# define SIZE_T MPI_UNSIGNED
+#elif SIZE_MAX == ULONG_MAX
+unsigned long SIZE_T_var;
+# define SIZE_T MPI_UNSIGNED_LONG
+#elif SIZE_MAX == ULLONG_MAX
+unsigned long long SIZE_T_var;
+# define SIZE_T MPI_UNSIGNED_LONG_LONG
+#else
+# error "Unknown SIZE_MAX."
+#endif
+
+
+int main(int argc, char *argv[])
+{
+  MPI_Init(&argc, &argv);
+
+  PACK_EXTERNAL(MPI_DOUBLE)
+  PACK_EXTERNAL(MPI_UNSIGNED)
+  PACK_EXTERNAL(MPI_INT)
+  PACK_EXTERNAL(MPI_BYTE)
+  PACK_EXTERNAL(MPI_CHAR)
+  PACK_EXTERNAL(MPI_PACKED)
+  PACK_EXTERNAL(SIZE_T)
+
+  MPI_Finalize();
+  return 0;
+}