Simplify implementation of mt_async for Julia >= v1.4

Author: oschulz

src/onthreads.jl

@@ -207,19 +207,23 @@ Equivalent to `Base.@async` on Julia <= v1.2, equivalent to
 `Base.Threads.@spawn` on Julia >= v1.3.
 """
 macro mt_async(expr)
-    # Code taken from Base.@async and Base.Threads.@spawn:
-    thunk = esc(:(()->($expr)))
-    var = esc(Base.sync_varname)
-    quote
-        local task = Task($thunk)
-        @static if VERSION >= v"1.3.0-alpha.0"
-            task.sticky = false
-        end
-        if $(Expr(:isdefined, var))
-            push!($var, task)
+    @static if VERSION >= v"1.4.0"
+        :(Base.Threads.@spawn $(esc(expr)))
+    else
+        # Code taken from Base.@async and Base.Threads.@spawn:
+        thunk = esc(:(()->($expr)))
+        var = esc(Base.sync_varname)
+        quote
+            local task = Task($thunk)
+            @static if VERSION >= v"1.3.0-alpha.0"
+                task.sticky = false
+            end
+            if $(Expr(:isdefined, var))
+                push!($var, task)
+            end
+            schedule(task)
+            task
         end
-        schedule(task)
-        task
     end
 end
 export @mt_async