package imagemagick: fix freetype detection, add test
package openexr: add link test
package imagemagick: fix runtime errors due to threading bugs

Runtime tests have revealed that libMagick++ crashes in pthread-related code.
Therefore, this patch disables threading in the package. Unfortunately the
configure switch is not enough; the resulting config header needs to be tweaked.
doc update: add note about installing cmake from backports on Debian Squeeze
package ilmbase: additional SHELL variable for configure