Support building without deprecated extensible-exceptions package

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Version 3001.1.7.5

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Merge branch 'maint'

Conflicts:
	Network/CGI/Monad.hs
	cgi.cabal
Version 3001.1.8.4

Signed-off-by: Anders Kaseorg <andersk@mit.edu>