test/pio_write_run: skip test if grib format is disabled

tests/pio_write_run.in

 #! @SHELL@
 set -e
 LOG="${LOG-pio_write.log}"
-mpi_task_num="${mpi_task_num-4}"
-suffix="${suffix-grb}"
-exec 5>&1 6>&2 >"$LOG" 2>&1
-../libtool --mode=execute \
-  @MPI_LAUNCH@ \
-  -n ${mpi_task_num} ${tool_wrap} ./pio_write ${pio_write_args}
-exec 2>&6 1>&5 5>&- 6>&-
-../libtool --mode=execute \
-  ${tool_wrap} ./cksum_read example_0.${suffix} example_0.cksum
-../libtool --mode=execute \
-  ${tool_wrap} ./cksum_read example_1.${suffix} example_1.cksum
-\rm "$LOG"
+if [ @ENABLE_GRIB@ = yes ]; then
+  mpi_task_num="${mpi_task_num-4}"
+  suffix="${suffix-grb}"
+  exec 5>&1 6>&2 >"$LOG" 2>&1
+  ../libtool --mode=execute \
+    @MPI_LAUNCH@ \
+    -n ${mpi_task_num} ${tool_wrap} ./pio_write ${pio_write_args}
+  exec 2>&6 1>&5 5>&- 6>&-
+  ../libtool --mode=execute \
+    ${tool_wrap} ./cksum_read example_0.${suffix} example_0.cksum
+  ../libtool --mode=execute \
+    ${tool_wrap} ./cksum_read example_1.${suffix} example_1.cksum
+  \rm "$LOG"
+else
+  # skip tests for unsupported formats
+  exit 77
+fi