yacc tool updates

The yacc tool now recognizes the bison syntax of using --header, --defines
and --graph options in YACCFLAGS without supplying an option argument.
Note that byacc doesn't allow leaving off the option argument, and would
error out in such cases.

Fixes #4326

The yacc tool now recognizes -H as a partial synonym for --header.
It only works in the no-option-argument form - giving an option argument
will likely break things.

Fixes #4327

Tests are expanded to cover these.

The graph file name (-g) is now generated relative to the requested
target file name, not to the source name, to match actual current
behavior. This is rare case - only kicks in if target explicitly requested
with a different base name than source - in this case SCons would emit a
different name than bison produces.  Unlikely to affect any real usage -
you would have to set up the condition described, and additionally use
the graph file as a source for some other build target.

The default file suffix for graph files is set to .gv, which is current
for Bison as of 3.8.  The previous suffix for Bison, dating back to 2.4,
was .dot, which is still the suffix byacc uses.  Docs updated to better
describe when you need to set this. The former default, .vcg, has not
been used since 2006.

Documentation updated for the two header suffix construction variables
to better describe (I hope) what these mean and when to set them. Other
doc updates as well.

Signed-off-by: Mats Wichmann <mats@linux.com>

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