configure: simplify set_default function

Originally committed as revision 15911 to svn://svn.ffmpeg.org/ffmpeg/trunk

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
 }