I noticed that the mesa-demos tarball contains *many* demos which are
probably unneeded by most users, and that the Debian mesa-utils package
merely installs 4 tools.  (The reason I wanted to package this in first
place was just to get glxinfo(1), which is one of these four.)  So I
copied that solution after all.

Updated patches below.  The glproto propagated input is not in any .pc
file so I positioned the comment accordingly.  I don't know why glproto
is propagated; maybe it's obvious for someone who knows X.Org and Mesa
well / the original packager.