don't reset MAKEFLAGS in recursive make in 'make fast'

It breaks some tests on Windows, because the stage=2 setting is lost
in the recursive call.  I don't understand why these variables are
passed in MAKEFLAGS on Windows (cygwin) but not on Unix, something to
do with environment variables presumably.

testsuite/mk/test.mk

@@ -150,5 +150,5 @@ accept:
 	$(MAKE) accept=YES
 
 fast:
-	$(MAKE) fast=YES MAKEFLAGS=
+	$(MAKE) fast=YES