SDL2_gfx: fix underlinking against -lm

Without the change `freeciv_sdl2` fails to detec `-lSDL2_gfx` as
https://hydra.nixos.org/build/294735307:

    checking for rotozoomSurface in -lSDL2_gfx... no
    configure: error: specified client 'sdl2' not configurable (SDL2_gfx is needed (www.libsdl.org))

pkgs/by-name/sd/SDL2_gfx/package.nix

@@ -38,6 +38,10 @@ stdenv.mkDerivation (finalAttrs: {
     "dev"
   ];
 
+  # Missing 'sincos()' implementation fails linking projects
+  # like 'freeciv_sdl2'.
+  env.NIX_LDFLAGS = "-lm";
+
   configureFlags = [
     (lib.enableFeature enableMmx "mmx")
     (lib.enableFeature enableSdltest "sdltest")