RFC: Pluggable Engines #2 (No Python, just evolution of current configure shell and make code)

The following commits allow engines to be added to the build, just by dropping them into folders in engines/ 

This is achieved by the engine providing three files, configure, engine.mk and engine-plugin.h which each contain the engine relevant code/data previously found in engines/configure.engines, engines/engines.mk and engines/plugins_table.h respectively.

Modifications to the configure script and makefiles cope with scanning the engine/ directory for folders containing these and picking up their contents.

No further build dependency changes are needed, though create_project will need a minor modification to cope with the removal of engines/configure.engines and migration to the same kind of scan for engines/*/configure as the configure script now does...

This is functional, but I don't consider it final. I am open to modifications, naming changes and even if merged, my changes do not block us moving in the future to using CSV or JSON in the engine folders, though I haven't used either currently.