sdl2-compat: symlink package config specification file into sdl2.pc

Previously, replacing SDL2 with sdl2-compat with dependents on the SDL2 pkgconfig file would just break.
An example for this breakage is pkgs.ffmpeg.override { SDL2 = pkgs.sdl2-compat; }.
This is fixed by symlinking the missing sdl2.pc file to point to sdl2_compat.pc.

This is analogous to #388304 (aa8bb53991)

pkgs/by-name/sd/sdl2-compat/package.nix

@@ -40,9 +40,7 @@ stdenv.mkDerivation (finalAttrs: {
     libX11
   ];
 
-  checkInputs = [
-    libGL
-  ];
+  checkInputs = [ libGL ];
 
   outputs = [
     "out"
@@ -67,6 +65,12 @@ stdenv.mkDerivation (finalAttrs: {
   patches = [ ./find-headers.patch ];
   setupHook = ./setup-hook.sh;
 
+  postFixup = ''
+    # allow as a drop in replacement for SDL2
+    # Can be removed after treewide switch from pkg-config to pkgconf
+    ln -s $dev/lib/pkgconfig/sdl2_compat.pc $dev/lib/pkgconfig/sdl2.pc
+  '';
+
   passthru = {
     tests =
       let