SDL2_ttf_2_0_15: init

This is an older version of the SDL2_ttf derivation needed for
`linthesia` to run properly. I don't know why a minor version change
broke things (2.0.15 -> 2.0.18) but it did and this fixes that without
disrupting other packages.

pkgs/development/libraries/SDL2_ttf/2.0.15.nix

···
+{ lib, stdenv, pkg-config, darwin, fetchurl, SDL2, freetype, libGL }:
+
+stdenv.mkDerivation rec {
+  pname = "SDL2_ttf";
+  version = "2.0.15";
+
+  src = fetchurl {
+    url = "https://www.libsdl.org/projects/SDL_ttf/release/${pname}-${version}.tar.gz";
+    sha256 = "0cyd48dipc0m399qy8s03lci8b0bpiy8xlkvrm2ia7wcv0dfpv59";
+  };
+
+  configureFlags = lib.optional stdenv.isDarwin "--disable-sdltest";
+
+  nativeBuildInputs = [ pkg-config ];
+
+  buildInputs = [ SDL2 freetype libGL ]
+    ++ lib.optional stdenv.isDarwin darwin.libobjc;
+
+  meta = with lib; {
+    description = "SDL TrueType library";
+    platforms = platforms.unix;
+    license = licenses.zlib;
+    homepage = "https://www.libsdl.org/projects/SDL_ttf/";
+  };
+}

pkgs/top-level/all-packages.nix

···
 
   SDL2_ttf = callPackage ../development/libraries/SDL2_ttf { };
 
+  SDL2_ttf_2_0_15 = callPackage ../development/libraries/SDL2_ttf/2.0.15.nix { };
+
   sdnotify-wrapper = skawarePackages.sdnotify-wrapper;
 
   sdrplay = callPackage ../applications/radio/sdrplay {};