When deleting obsolete generated files (monstr.c, vis_tab.[ch])
for src 'make clean' or clean called from src 'make spotless' or
from top Makefile, don't complain if they aren't present.
Deletion failure wasn't stopping 'make' but did result in looking
like something might be wrong.
Before:
rm monstr.c vis_tab.c ../include/vis_tab.h #obsolete generated files
rm: monstr.c: No such file or directory
rm: vis_tab.c: No such file or directory
rm: ../include/vis_tab.h: No such file or directory
make[1]: [clean] Error 1 (ignored)
After:
rm -f monstr.c vis_tab.c ../include/vis_tab.h #obsolete generated files
(no extra feedback from rm or make).
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