rewrite Makefile to handle multiple rgbasm objects

for now, any new files to preprocess will trigger the targets for all objects.
this isn't a problem yet since there's still only one object.
change old .tx includes to .asm

these were forgotten about with the last preprocessor change.
this is required for object dependencies to be handled by make.
scan source files for Makefile dependencies

preprocessing should work with multiple object files now
rename make variables for graphics files
rename lzs make target to gfx
abstract make targets are now phony