Fix bug #9874 where MRWs showed in purple
As it turns out the 270cw rotation was wrong. Made the code more explicit as to what it is doing with the three flip bits.
Always specify requred instruction set. Reported by bartokk
Allow compilation with non-ASCII chars in the path

The problem was that we put the absolute path into darktableconfig.xml
to find the DTD. This however failed when there were some special
characters in the path. Now we just copy darktableconfig.dtd to build.
Merge pull request #508 from pedrocr/fix-purple-mrw

Fix bug #9874 where MRWs showed in purple