call freeze_support() unconditionally since it's a NOOP on non-relevant platforms

src/gam.py

@@ -14970,8 +14970,7 @@ def ProcessGAMCommand(args):
 
 # Run from command line
 if __name__ == "__main__":
-  if sys.platform.startswith('win'):
-    freeze_support()
+  freeze_support()
   if sys.version_info[0] < 3 or sys.version_info[1] < 5:
     systemErrorExit(5, 'GAM requires Python 3.5 or newer. You are running %s.%s.%s. Please upgrade your Python version or use one of the binary GAM downloads.' % sys.version_info[:3])
   sys.exit(ProcessGAMCommand(sys.argv))