SDL2: Keep .a files on dontDisableStatic; don't move them to $dev; prune .la.

Most other packages don't move `.a` files to "$dev", and that is because
it makes the pkg-config `.pc` file wrong (the `libdir` is the non-dev one).

Keeping them in the main output makes static linking of SDL2 work.

See added comment about pruning of `.la` files.

pkgs/development/libraries/SDL2/default.nix

@@ -75,9 +75,21 @@ stdenv.mkDerivation rec {
     ++ optional alsaSupport "--with-alsa-prefix=${alsaLib.out}/lib"
     ++ optional stdenv.isDarwin "--disable-sdltest";
 
+  # We remove libtool .la files when static libs are requested,
+  # because they make the builds of downstream libs like `SDL_tff`
+  # fail with `cannot find -lXext, `-lXcursor` etc. linker errors
+  # because the `.la` files are not pruned if static libs exist
+  # (see https://github.com/NixOS/nixpkgs/commit/fd97db43bcb05e37f6bb77f363f1e1e239d9de53)
+  # and they also don't carry the necessary `-L` paths of their
+  # X11 dependencies.
+  # For static linking, it is better to rely on `pkg-config` `.pc`
+  # files.
   postInstall = ''
-    moveToOutput lib/libSDL2main.a "$dev"
+    if [ "$dontDisableStatic" -eq "1" ]; then
+      rm $out/lib/*.la
+    else
       rm $out/lib/*.a
+    fi
     moveToOutput bin/sdl2-config "$dev"
   '';