Thanks to Robert Merkel for reporting them.
* Build gnucash-design.info if makeinfo is available, but don't complain
if it isn't.
* Fail Cmake if LibXSLT and xsltproc aren't found. Those are required to
build the gschemas and GnuCash won't work without them.
* Fix the WebKit detection so that it isn't faked out into thinking it has
WebKit2Gtk-4.0 when it's WebKit2Gtk-3.0 and cmake is run with a
CMakeCache.txt present.
John Ralls