configure: make sh_quote() more robust

Originally committed as revision 24376 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -304,7 +304,7 @@ c_escape(){
 
 sh_quote(){
     v=$(echo "$1" | sed "s/'/'\\\\''/g")
-    test "$v" = "${v#*[ |&;<>()$\`\\\"\'*?\[\]#~=%]}" || v="'$v'"
+    test "x$v" = "x${v#*[^A-Za-z0-9_/.+-]}" || v="'$v'"
     echo "$v"
 }