Completely re-wrote the Makefiles after reading `info make` for two hours. Some of...

Completely re-wrote the Makefiles after reading `info make` for two hours. Some of the changes were:
    - adding .PHONY targets in;
    - moving all targets related to actually running xuni into the root Makefile;
    - replacing backquotes with $(shell ...);
    - included the file depend instead of adding the dependency list manually;
    - renamed gccmm to depend and modified it so that it is fully automated (once you run it);
    - defining OBJECTS as $(SOURCES:.c=.o).
And so much more.

A Makefile is yet to be added to doc/, but that can wait.

Also added some information about the GPL into the top of xuni.c (the \mainpage comment), taken almost verbatim from COPYING.