fix Xcode concurrency code

src/pch.h

@@ -36,6 +36,7 @@
     #include <sys/stat.h>
     #include <sys/types.h>
     #include <dirent.h>
+    #include <dispatch/dispatch.h>
 
 #elif __ANDROID__
 
@@ -50,6 +51,7 @@
 
     #define SHADER_VERSION "#version 300 es\n"
     #define PP_OS "android"
+    #define __block
 
 #elif _WIN32
 
@@ -70,6 +72,7 @@
 
     #define SHADER_VERSION "#version 150\n"
     #define PP_OS "win"
+    #define __block
 
 #endif