Add @nogc UDA, fix some small issues, xmake changes

-add @nogc to some functions where it was missing
-fix compilation issue in mallocator
-fix Meson build
-some work on xmake build

source/bubel/ecs/std.d

@@ -279,7 +279,7 @@ static struct Mallocator
 
         static if (__traits(isPOD, T))
         {
-            __gshared immutable T init = T.init;
+            __gshared T init = T.init;
             memcpy(ret, &init, T.sizeof);
         }
         else static if (is(T == struct))