Only create symlinks for external gempak if they don't exist

`ln -sf` wasn't overwriting existing links, but we don't need to do
the operation if the symlink already exists anyway.

gempak/ush/gdas_ecmwf_meta_ver.sh

@@ -44,7 +44,9 @@ for area in ${areas}; do
         dgdattim=$(printf "f%03d" "${fhr}")
         sdatenum=$(date --utc +%y%m%d -d "${PDY} ${cyc2} - ${fhr} hours")
 
-        ln -sf "${COMINecmwf}/ecmwf.20${sdatenum}/gempak" "ecmwf.20${sdatenum}"
+        if [[ ! -L "ecmwf.20${sdatenum}" ]]; then
+            ln -sf "${COMINecmwf}/ecmwf.20${sdatenum}/gempak" "ecmwf.20${sdatenum}"
+        fi
         gdfile="ecmwf.20${sdatenum}/ecmwf_glob_20${sdatenum}12"
 
         # 500 MB HEIGHT METAFILE

gempak/ush/gdas_ukmet_meta_ver.sh

@@ -49,7 +49,9 @@ for area in ${areas}; do
         sdatenum=${stime:0:6}
         cyclenum=${stime:6}
 
-        ln -sf "${COMINukmet}/ukmet.20${sdatenum}/gempak" "ukmet.20${sdatenum}"
+        if [[ -L "ukmet.20${sdatenum}" ]]; then
+            ln -sf "${COMINukmet}/ukmet.20${sdatenum}/gempak" "ukmet.20${sdatenum}"
+        fi
         gdfile="ukmet.20${sdatenum}/ukmet_20${sdatenum}${cyclenum}${dgdattim}"
 
         # 500 MB HEIGHT METAFILE