configure: add foo_select facility

If an option 'foo' is enabled, all options listed in foo_select
are automatically enabled, unless explicitly disabled.  In the
latter case, foo is also disabled.

Originally committed as revision 15684 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -336,17 +336,20 @@ check_deps(){
 
         eval dep_all="\$${cfg}_deps"
         eval dep_any="\$${cfg}_deps_any"
+        eval dep_sel="\$${cfg}_select"
 
-        pushvar cfg dep_all dep_any
-        check_deps $dep_all $dep_any
-        popvar cfg dep_all dep_any
+        pushvar cfg dep_all dep_any dep_sel
+        check_deps $dep_all $dep_any $dep_sel
+        popvar cfg dep_all dep_any dep_sel
 
         enabled_all $dep_all || disable $cfg
         enabled_any $dep_any || disable $cfg
+        disabled_any $dep_sel && disable $cfg
 
         if enabled $cfg; then
             eval dep_extralibs="\$${cfg}_extralibs"
             test -n "$dep_extralibs" && add_extralibs $dep_extralibs
+            enable $dep_sel
         fi
 
         disable ${cfg}_checking