mednafen: Create a new recipe.

Compiles/installs/starts, but crashes in an OpenGL blitter on my end.
Not sure what the culprit is.

app-emulation/mednafen/patches/mednafen-unistd.patch

@@ -0,0 +1,26 @@
+From b22df22583fef8b9100e239de6184af1f6863de5 Mon Sep 17 00:00:00 2001
+From: waddlesplash <waddlesplash@gmail.com>
+Date: Sun, 6 Mar 2016 12:42:53 -0500
+Subject: [PATCH] mempatcher: Include unistd.h, as unlink(3) is used.
+
+Some systems (e.g. Haiku) which have strict(er) POSIX implementations
+require this.
+---
+ src/mempatcher.cpp | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/src/mempatcher.cpp b/src/mempatcher.cpp
+index a7e58ca..1c41b8d 100644
+--- a/src/mempatcher.cpp
++++ b/src/mempatcher.cpp
+@@ -27,6 +27,7 @@
+ #include <ctype.h>
+ #include <trio/trio.h>
+ #include <errno.h>
++#include <unistd.h>
+ #include <vector>
+ 
+ #include "general.h"
+-- 
+2.7.0
+