Added make code to build the optimized versions.
Made the makefiles' clean target remove the object files from the source directory.
Create the makefile variable REFS in the same way that TESTS is made.
All regression-tests subdirectories are cleaned before the first test starts, so that 'make continue' won't see debris when it moves to the next directory.
Use simply-defined make variables.
Shortenned the code that creates the TESTS make variable.

And, fixed the variable-substitution that creates the CC65 optimization option.
Made sure that parallel-building the regression-tests doesn't start a test before all of the directories are clean.
Merge pull request #144 from greg-king5/regression-makes

Regression-tests makefiles