libretro.mame2016: fix build with gnumake-4.3

Builds currently fail with `ar` trying to operate on what are clearly
two paths concatenated together.  It stems from a backward-incompatible
change in Make:

> Previously appending using '+=' to an empty variable would result in
> a value starting with a space.  Now the initial space is only added
> if the variable already contains some value.  Similarly, appending an
> empty string does not add a trailing space.

This issue was first reported on the MAME repository proper
(mamedev/mame#6248), and affects libretro's
2016 snapshot as well.  A fix that is reported to work with previous
versions of Make was upstreamed to:
- GENie, the build system: bkaradzic/GENie#493
- MAME: mamedev/mame#6262
- libretro: libretro/mame2016-libretro#47

The fetched patch comes from the last of these.

pkgs/misc/emulators/retroarch/cores.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchgit, fetchFromGitHub, fetchFromGitLab, cmake, pkgconfig, makeWrapper, python27, python37, retroarch
+{ stdenv, fetchgit, fetchFromGitHub, fetchFromGitLab, fetchpatch, cmake, pkgconfig, makeWrapper, python27, python37, retroarch
 , alsaLib, fluidsynth, curl, hidapi, libGLU, gettext, glib, gtk2, portaudio, SDL, SDL_net, SDL2, SDL2_image, libGL
 , ffmpeg_3, pcre, libevdev, libpng, libjpeg, libzip, udev, libvorbis, snappy, which, hexdump
 , miniupnpc, sfml, xorg, zlib, nasm, libpcap, boost, icu, openssl
@@ -630,6 +630,13 @@ in with stdenv.lib.licenses;
       rev = "02987af9b81a9c3294af8fb9d5a34f9826a2cf4d";
       sha256 = "0gl7irmn5d8lk7kf484vgw6kb325fq4ghwsni3il4nm5n2a8yglh";
     };
+    patches = [
+      (fetchpatch {
+        name = "fix_mame_build_on_make-4.3.patch";
+        url = "https://github.com/libretro/mame2016-libretro/commit/5874fae3d124f5e7c8a91634f5473a8eac902e47.patch";
+        sha256 = "061f1lcm72glksf475ikl8w10pnbgqa7049ylw06nikis2qdjlfn";
+      })
+    ];
     description = "Port of MAME ~2016 to libretro";
     license = gpl2Plus;
     extraNativeBuildInputs = [ python27 ];