Don't use non-portable "-n" option to test

This commit was SVN r5510.

config/ompi_find_type.m4

@@ -48,7 +48,7 @@ for type in $types; do
 
         # If the size matches the target size, we're done
 
-        if test -n "$target_size" -a \
+        if test "$target_size" != "" -a \
             "$type_size" = "$target_size"; then
             real_type="`echo $type | sed -e 's/:/ /'g`"
         fi