Fix build-system bug in configuration and installation of
stylesheet.css for the case where the documentation is prebuilt.
Tested by Alan W. Irwin <firstname.lastname@example.org> by running
scripts/make_tarball.sh -c -i /tmp/plplot_install
find /tmp/plplot-dist-prep -name "*.out" |xargs grep -i error
and browsing the resulting documentation that was installed from the
generated preliminary version of the release tarball in
/tmp/plplot_install/share/doc/plplot/html while looking carefully
for stylesheet issues.