Factor out name of special rebuilding-heaps environment variable

The name of this variable is now specified in just one place.

Progress with github issue #105

POLYC + POLY_VERSION now available in Holmakefiles

Also change POLYC from string option to string in Systeml (and "" in
Moscow ML environments).

Put POLY_VERSION into Systeml.

This allows code that might be compiled with Moscow ML to also
work (there the POLY_VERSION value will be 0).

Use PolyML.SaveState instead of executable heaps

Interface in terms of "HOLHEAP" declarations in Holmakefiles stays the
same. Implementation uses hierarchical saved states. All command-line
executables should also now be compiled with polyc.

Closes #292

Make copy of portable pointer_eq available in Systeml.

This then allows regexpMatch to be portable too.

Allow user to configure location of dot tool for theory graph building.

The default is /usr/bin/dot, because that’s where it is on my Linux machine. If you need it to be somewhere else, create a config-override or poly-includes.ML file, depending on which ML you’re using.

PolyML compatibility. See tools-poly/README.