Added a bunch of information to the near bottom of HOWTO.INSTALL
on calc Makefiles. This information discusses the various
Makefiles found in the calc source.
Added comments in various calc Makefiles about their origin.
In particular, for Makefiles that are constructed such as
Makefile.simple, custom/Makefile and custom/Makefile.simple
there are comments about how they were made.
For all calc Makefiles, including those in sub-directories,
near the top there is now a line of the form:
# SRC: ... some message about the origin ...