Apparently sometimes when doing the distcheck the generator can't
open the file for writing (gets ENOACCESS) presumably because there's
already a file there that's read-only. So remove any existing
settings spec before regenerating.
gtk-doc removes the 'html' directory as part of the build process
so we need to have spec.html generation depend on the gtk-doc html
target. Also, fix up the paths to make sure the generated spec.html
gets copied into the final 'html' directory.
