SDL2_gfx: fix underlinking against -lm (#400295)

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")
@@ -45,6 +49,8 @@ stdenv.mkDerivation (finalAttrs: {
 
   strictDeps = true;
 
+  enableParallelBuilding = true;
+
   passthru = {
     tests.pkg-config = testers.hasPkgConfigModules {
       package = finalAttrs.finalPackage;