Implement 'scons srcdist=1' functionality, to build a gripped tar file of the source distribution

In slight contrast to the Makefile methodology, the gzipped tar file is created in a new subdirectory called ~/dist

This is preferable in SCons, as now the build dependency for the tar file is handled just like the dependencies for any other program that might be part of the build process.  That is, if the tar file does not exist, or any of its contents are changed, then a new tar file is built.  Otherwise, not.