Fix remaking of info subject pages

Fix remaking of info subject pages

We used to do all the info indexing work in info.pl: find subjects,
create subjects.mk (to tell make the list of subjects), the subject
pages, and TOP.t.  Worked, but touching an info page triggered a full
rebuild of all subject pages and TOP.t.

Commit 2f14064 (v4.3.0) tried to avoid that by splitting info.pl into
findsubj.pl, mksubj.pl, mktop.pl.  findsubj.pl puts not just the
subjects into subjects.mk, but also make rules for the subject pages,
to guide their remaking.  mksubj.pl creates a single subject page.
mktop.pl creates TOP.t.

Unfortunately, this doesn't work so well.  Since subjects.mk doesn't
exist in a virgin tree, we use -include.  Unwanted consequence:
findsubj.pl failure doesn't stop make.  Moreover, the complex make
machinery breaks down when info sources get removed or subjects get
dropped.

Go back to the old method, except keep mktop.pl separate, as that part
works just fine, and use simpler make rules.  mksubj.pl now creates
subjects.mk and all subject pages, like info.pl did.

This effectively reverts most of commit 2f14064.  I'll address the
excessive rebuilding of subject pages in a different way shortly.