We already depend on automake 1.9, so using the replacement of
INCLUDES (AM_CPPFLAGS) won't hurt, especially now that it is
a warning shown by default since automake 1.13, as evidenced by
this build log of mine:

Makefile.am: installing './INSTALL'
eel/Makefile.am:5: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
eel/Makefile.am: installing './depcomp'
parallel-tests: installing './test-driver'
libnautilus-extension/Makefile.am:7: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
libnautilus-private/Makefile.am:5: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
nautilus-sendto-extension/Makefile.am:3: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
src/Makefile.am:13: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
test/Makefile.am:3: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')

Signed-off-by: Alexander Lar...