Keep this disabled by default (since it takes a fair amount of time
and has good test coverage upstream), but including it in disabled
form so that it's easy to enable temporarily for testing occasionally.
Libcxx does test the mingw configuration in their own CI (with an older
pinned release of the compiler), so this is mostly redundant, and
primarily relevant for testing that the very latest compiler (and
python setup) works for the tests.
Martin Storsjö
c49460aa86