BUG #13511

Fixed SETUP.sh to not overwrite prefix_configs, making 'make install' 
install in "/"


BUILD/SETUP.sh:
  BUG #13511
  
  test -n is for testing if a FILE is zero length! Plus, if the value is zero,
  we don't want to set it to that value, we want to leave it as it was 
  already set. We _want_ to set it _if_ it is _not_ zero. Changed to "! -z",
  which is for testing strings.

BUILD/SETUP.sh

@@ -10,7 +10,7 @@ prefix_configs="--prefix=/usr/local/mysql"
 just_print=
 just_configure=
 full_debug=
-if test -n $MYSQL_BUILD_PREFIX
+if test ! -z $MYSQL_BUILD_PREFIX
 then
   prefix_configs="--prefix=$MYSQL_BUILD_PREFIX"
 fi