Enable gcc -Wold-style-declaration and fix issues
Store fread result in variable with correct type - size_t
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.