!13 change expression calculation to variable calculation

Merge pull request !13 from lishifu/master

configure

@@ -3317,7 +3317,7 @@ fi
 
 # this expression is set up to avoid unnecessary integer overflow
 # blocksize is already guaranteed to be a factor of 1024
-RELSEG_SIZE=`expr '(' 1024 / ${blocksize} ')' '*' ${segsize} '*' 1024`
+RELSEG_SIZE=$[(1024 / ${blocksize}) * ${segsize} * 1024]
 { $as_echo "$as_me:$LINENO: result: ${segsize}GB" >&5
 $as_echo "${segsize}GB" >&6; }