One possible fix to work around the version of abseil in current debian testing

[paleolimbot]

(Just keeping track of this on a branch for now and also looking into another fix)

For #293.

[paleolimbot]

In its current form, this doesn't compile on Windows because:

      |   ^
s2/s2boolean_operation.cc: In member function 'bool S2BooleanOperation::Impl::CrossingProcessor::is_degenerate(S2BooleanOperation::Impl::ShapeEdgeId) const':
s2/s2boolean_operation.cc:1215:32: error: 'const class std::unordered_map<s2shapeutil::ShapeEdgeId, bool>' has no member named 'contains'
 1215 |     return is_degenerate_hole_.contains(a_id);
      |                                ^~~~~~~~
make: *** [C:/R/etc/x64/Makeconf:294: s2/s2boolean_operation.o] Error 1
ERROR: compilation failed for package 's2'
* removing 'D:/a/s2/s2/check/s2.Rcheck/s2'\

I think contains is C++20+