Add CXX11 back

[paleolimbot]

For #232...I also have a winbuilder check queued because I realized there wasn't CXX_STD = CXX11 in Makevars.ucrt (this matters for OBJECTS, so maybe it matters for CXX_STD too).

[codecov-commenter]

Codecov Report

Merging #234 (35fa7cb) into main (a47db3e) will not change coverage.
The diff coverage is n/a.

❗ Current head 35fa7cb differs from pull request most recent head 0875a17. Consider uploading reports for the commit 0875a17 to get more accurate results

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@@           Coverage Diff           @@
##             main     #234   +/-   ##
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  Coverage   94.04%   94.04%           
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  Files          46       46           
  Lines        3524     3524           
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  Hits         3314     3314           
  Misses        210      210           

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[paleolimbot]

@edzer this seems to work (but we do get a NOTE about CXX11): https://win-builder.r-project.org/h4Sa5M1JoSWA/00check.log

[edzer]

Great! Given how close we are to our final deadline, I suggest I continue with submitting to CRAN now.