fix clobbering and CFLAG problems

Apparently .SUFFIX can be used to prevent yacc from overwriting delbrot.c.
There was also a problem with delbrot.l and internalfilters.c not being
recompiled when switching between "make"/"make mkvsynth" and "make delbrot".
This must be done to ensure that the -DDELBROT flag is processed. These
files will now be forcibly recompiled every time make is run.