Docs: add a 'pdf' cmake target.

We still don't build or ship a PDF PuTTY manual by default, but we may
as well conveniently expose Halibut's ability to do so.

(I don't guarantee the resulting PDF is particularly pretty -- some of
our overlong code lines do go off the right margin currently.)

(cherry picked from commit f9a8213d95)

doc/blurb.but

@@ -23,6 +23,7 @@ page</a>.</p>}
 \cfg{winhelp-filename}{putty.hlp}
 \cfg{info-filename}{putty.info}
 \cfg{chm-filename}{putty.chm}
+\cfg{pdf-filename}{putty.pdf}
 
 PuTTY is a free (MIT-licensed) Windows Telnet and SSH client. This
 manual documents PuTTY, and its companion utilities PSCP, PSFTP,