cross stdenv adaptor: Support --host --build --target across the board

Packages get --host and --target by default, but can explicitly request
any subset to be passed as needed. See docs for more info.

rustc: Avoid hash breakage by using the old (ignored)
dontSetConfigureCross when not cross building

pkgs/development/libraries/libav/default.nix

@@ -105,7 +105,7 @@ let
     installCheckTarget = "check"; # tests need to be run *after* installation
 
     crossAttrs = {
-      dontSetConfigureCross = true;
+      configurePlatforms = [];
       configureFlags = configureFlags ++ [
         "--cross-prefix=${stdenv.cross.config}-"
         "--enable-cross-compile"