libctf: workaround automake bug with conditional info pages

libctf: workaround automake bug with conditional info pages

It looks like automake makes assumptions about its ability to build info
pages based on the GNU standard behavior of shipping info pages with the
distributions.  So even though the info pages were conditionalized, and
automake disabled some of the targets, it was still creeping in by way
of unconditional INFO_DEPS settings.

We can workaround this by adding a stub target for the info page when
building info pages are disabled.  This tricks automake into disabling
its own extended generation target.  I'll follow up with the automake
folks to see what they think.