[boost] Request to merge fixes to docca

I would like to merge the following commits: https://github.com/boostorg/docca/commit/0ce4e198398dbb52f1de0029f4ed9b7d2bd... https://github.com/boostorg/docca/commit/8ba36c0c1e1adb32dbf4cd9c7f3cb36f339... https://github.com/boostorg/docca/commit/4b61682742d6fc0d373698d426350240881... To address these issues: https://github.com/boostorg/docca/issues/136 https://github.com/boostorg/docca/issues/105 These fix assorted compatibility issues with doxygen 1.8.16 and newer