Short overview of FriCAS build process.

Normal build from git repo works as follows:

1) First is src/lisp subdirectory.  This creates 'lisp' executable
   which is kind of virtual Lisp specialized to support FriCAS.
   In particular it contains Lisp packages, utility functions
   for creating executables and various Lisp macros and functions
   for supporting Shoe and Spad.  Basically, thanks to our
   functionality added to 'lisp' in other stages we have
   resonably uniform set of tools.
2) src/boot.  This contains Shoe to Lisp translator.  Uses bootstrap
   techinque to build from cached (precompiled) Lisp files and
   then re-build from Shoe sources.  Part titled 'The Boot Compiler'
   in src/boot/Makenotes.tex contains reasonable description of
   bootstrap process for Shoe.  The result of Shoe bootstrap
   is 'bootsys'.
3) src/interp.  Here we use 'bootsys' to compile '.boot' files.
   Things are somewhat complicated because we want to include
   Shoe to Lisp translator in 'interpsys' and 'FRICASsys'.
   Also ECL is different than other Lisps.  That is why we
   first create 'makeint.lisp' from the Makefile and delegate
   creation of executable to 'makeint.lisp'.  The result
   is 'interpsys'.
4) 'interpsys' is used to compile Spad files and bootstrap
   algebra.  This is described in 'doc/algebra_build.txt'.
5) Once algebra is built, a small but frequently used part
   of algebra is preloaded into 'interpsys' creating
   'FRICASsys' (the rest of algebra is loaded on demand).
   Also a few variables in 'FRICASsys' have different values
   than in 'interpsys'.  For ECL preloading does not work
   and 'FRICASsys' differs from 'interpsys' only in a few
   variables.
6) FRICASsys is used to generate .pht pages for HyperDoc.

Release tarball contains pregenerated Lisp files for algebra,
databases and .pht pages so in stage 4 there is only Lisp
compilation and stage 6 is completely skipped.
