add a "preload check" for examples/sharedsystem/setup.xtm

it's not a proper aot-able library thing (although that might be nice in the
future) but it should prevent issues with folks accidentally double-evaling the
load command for the sharedsystem setup file

examples/sharedsystem/setup.xtm

@@ -27,9 +27,14 @@
 
 ;;; Code:
 
+(sys:load-preload-check 'sharedsystem-setup)
+(define *xtmlib-sharedsystem-setup-loaded* #t)
+
 (sys:load "examples/sharedsystem/midisetup.xtm")
 (sys:load "examples/sharedsystem/audiosetup.xtm")
 
+(define *xtmlib-sharedsystem-setup-loaded* #t)
+
 (print-with-colors
  'black 'green #t
  (print "shared system successfully loaded\n"))