Edit detail for FriCASPreGenerated revision 2 of 3

This is a description of how to generate and how to use the new
'pre-generated' system.

How to generate
---------------

The FriCAS build system comes with a new make target "dist" which
creates a full source tarball together with pre-generated
machine-independent data that speed up the build process.

The command "make dist" has to be executed in an out-of-source build
after completion of a full successful build. Furthermore, the source
tree must be a clean checkout from the repository (SVN or GIT), since
all files in this tree will be copied into the distribution.

It creates a file fricas-VERSION.tar.bz2 where VERSION corresponds
to the second argument of AC_INIT in configure.ac.

Currently, one cannot simply call "make dist", but must provide a
location for the NOWEB tarball as well as the directory of
(not-yet-generated) .help files. The typical invocation is thus:

  make NOWEB=/path/to/noweb.tar.bz2 helpsrcdir=/path/to/spadhelp dist

The generated tarball will contain a "zips" subdirectory with the noweb
tarball and a "pre-generated" subdirectory with all the
machine-independent pre-generated files.

How to use
----------

FriCAS can be configured to used machine-independent files from an
earlier compilation.

In short, the Makefile variable PREGENERATED must either be empty
(then no pre-generated data is used) or contain the absolute path of
the directory of pre-generated data.

There are several ways to let the FriCAS build system know about where
the pre-generated data is located. In each case the build system
assumes that the provided data is sane and does not explicitly check.

1) If there is a subdirectory "pre-generated" inside the FriCAS source
   tree, then configure will automatically set PREGENERATED to point
   to this directory.
2) If configure is called with
   --with-pre-generated=/absolute/path/to/pre-generated, then
   PREGENERATED is set  to /absolute/path/to/pre-generated.
3) One can call make like
     make PREGENERATED=/absolute/path/to/pre-generated
   which does the obvious thing.