Error/warning refactoring.

Now we only test for each potential problem once and throw the
appropriate error depending upon which goal(s) are requested.
Allow compiling into program/library to be turned off.

By setting the MODE variable, the makefile can be restriced to compiling
source files into only a library or only a program or (as before) keep
the possibility of compiling them into both.
cleanall now explicitly removes the dependency directory.

This is in case someone sets it to be in a different location.
Minor layout rearranging.

Directory structure is now in the area where user modifications are
encouraged and including the arch file is not.
Correct help message for new target.
Raise warning/error if MODE is not set at all.
Make program and library suffixes and prefixes configuration settings.