make out of order read a fatal error

For png and jpg, out of order must always be a fatal error, since you
can't read from a specific y position.

This was a g_assert() before which meant that this test was missing from
production builds. It's now always a fatal error.
tiny polish
add thread_shutdown() to python

see https://github.com/jcupitt/libvips/issues/170
Prevent empty else block in configure

Enables libvips to compile on RHEL/Centos 6
Merge pull request #171 from lovell/remove-config-empty-else-7.40

Prevent the version of autotools in Redhat/Centos 6 generating empty else statements
bump version to 7.40.8
Merge branch '7.40'

Conflicts:
	ChangeLog
	configure.ac