configure: simplify set_default function

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15911 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -362,7 +362,7 @@ disabled_any(){
 
 set_default(){
     for opt; do
-        eval test -z "\$$opt" && eval $opt=\$${opt}_default
+        eval : \${$opt:=\$${opt}_default}
     done
 }