Since all.xml is a built file these days, and will contain paths
relative to $(top_srcdir)/introspection even though it's not in
that directory, it seems we need to clue xsltproc into where the
other API XML files really are.
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.
