Makefile: implement different compilation methods, rename _COMPILE_ to ORGCM
* default.mk, targets.mk, lisp/Makefile: Rename _COMPILE_ to ORGCM.
* default.mk: Define ORGCM=dirall, show other possible values as
comments
* targets.mk: Implement target `single´, temporarily set
ORGCM (overriding configuration). Show newly introduced
configuration variables with `check-*´.
* lisp/Makefile: Implement dispatch for compile based on ORGCM.
Output header that contains the target for easier debugging.
* Makefile: Document target `single´.