Doxygen: Set HAVE_DOT = NO

Debian and Ubuntu have a patch that changes the upstream default to
HAVE_DOT = YES. Undo it to have more consistent results across distros.

This was noticed in Ubuntu CI runner where "doxygen" tried to run "dot"
but that failed due to "dot" not being installed. "doxygen" still
finished with exit status 0 until the commit that turned warnings to
errors with WARN_AS_ERROR = FAIL_ON_WARNINGS.

doxygen/Doxyfile

@@ -39,3 +39,7 @@ PREDEFINED             = LZMA_API(type)=type \
                          tuklib_attr_noreturn= \
                          lzma_attribute(attr)= \
                          lzma_attr_alloc_size(size)=
+
+# Debian and Ubuntu patch Doxygen so that HAVE_DOT = YES is the default.
+# Set HAVE_DOT explicitly to get consistent behavior across distributions.
+HAVE_DOT               = NO