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/libvpx/default.nix

@@ -148,7 +148,7 @@ stdenv.mkDerivation rec {
     isCygwin = stdenv.cross.libc == "msvcrt";
     isDarwin = stdenv.cross.libc == "libSystem";
   in {
-    dontSetConfigureCross = true;
+    configurePlatforms = [];
     configureFlags = configureFlags ++ [
       #"--extra-cflags="
       #"--extra-cxxflags="

pkgs/development/libraries/libvpx/git.nix

@@ -156,7 +156,7 @@ stdenv.mkDerivation rec {
     isCygwin = stdenv.cross.libc == "msvcrt";
     isDarwin = stdenv.cross.libc == "libSystem";
   in {
-    dontSetConfigureCross = true;
+    configurePlatforms = [];
     configureFlags = configureFlags ++ [
       #"--extra-cflags="
       #"--prefix="