Implement recursive make to avoid compatibility problems

Hi Ben,

Lewis found some problems on his Mac with the current GNU autotools setup.  So, we thought it would be best to setup the build system in the standard way, which is compatible with most systems and autotools versions.  

The standard way is to use a recursive make, where each sub-directory contains it's own Makefile.  Besides the compatibility argument, this has the advantage that one does not need to include the header files like this

  `#include "src/header.h"`

but instead one can keep the old (standard) inclusion

  `#include "header.h"`

This also makes it easyer for other people to reuse the Softsusy code, because they will probably need to include the header files in the standard way.

All the changes in this pull request, implement the recursive make system.  The main change is that the src/ and doc/ directories now contain their own Makefile.  I also checked that `make distcheck` works properly.

Best,
Alex