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
fix mono <-> rgb converter extra band handling

see

https://github.com/jcupitt/libvips/issues/172

thanks James
Merge branch '7.40'
support exif res unit "none"

well why would you want that, but it seems some images use it
Merge branch '7.40'
draw and hough working in Python
finish python vips8
fix jfif resunit read

see https://github.com/jcupitt/libvips/issues/170

the previous fix just did exif resolution "none", we need jfif "none" as
well
Merge branch '7.40'

Conflicts:
	ChangeLog
	configure.ac
fix a setjmp() loop in pngread

see

https://github.com/jcupitt/libvips/issues/175

stupid setjmp()
support GRAY as an input and output ICC space

see

https://github.com/jcupitt/libvips/issues/176
oops, GRAY is an lcms2 feature
Merge branch '7.40'
add @background arg to save

used to flatten A out of images, if necessary

vipsthumbnail rtd-img.png -o x.jpg[background=255]