Portable Makefiles

[Glenn Morris]

It turns out that the doc/*/Makefiles have not been buildable without
GNU make for some time, due to (non-portable) use of $< in ordinary
rules. Probably my fault; I think I saw $< being used in other Emacs
Makefiles and did not appreciate that it was always in suffix rules.

Anyway, it's now fixed in the emacs-24 branch, but I think the lack of
outcry [1] is an interesting data-point with respect to possibly
requiring GNU make to build Emacs at some future date.

Perhaps pretesting did not turn this up because the info files are
pre-built in tarfiles.

[1] I can only find two reports:
http://lists.gnu.org/archive/html/help-gnu-emacs/2012-04/msg00074.html
http://groups.google.com/group/gnu.emacs.help/msg/3ce576d7a4621a09