libcxxrt: Add multiple outputs

Good to keep headers out of closure, even if there is just one.

pkgs/development/libraries/libcxxrt/default.nix

@@ -13,9 +13,11 @@ stdenv.mkDerivation {
 
   nativeBuildInputs = [ cmake ];
 
+  outputs = [ "out" "dev" ];
+
   installPhase = ''
-    mkdir -p $out/include $out/lib
-    cp ../src/cxxabi.h $out/include
+    mkdir -p $dev/include $out/lib
+    cp ../src/cxxabi.h $dev/include
     cp lib/libcxxrt${stdenv.hostPlatform.extensions.library} $out/lib
   '';