Adding support for OpenSlide 3.4.0 and fixing the ImageMagick update craziness forever

OpenSlide 3.4.0, although only a minor version, actually adds several new features that are of note -- several new WSI formats are now supported, including Ventana BIF.  In order to compile it, though, SQLite 3.6+ is required.  Also, as we've discussed before, ImageMagick's wonky method of updating its libraries has caused problems in the past, and now that 6.8.7-10 has made it into the legacy/ directory, why not just permanently use it?

I can confirm compilation of libvips 7.38 on my win32 build virtual machine with these changes, and can confirm full functionality of the resultant binaries on my windows machine.