We don't seem to have a build on MS using ICU

Early in 2017, MS admitted their APIs for accessing timezone information were not fit for purpose and since then they've advocated using ICU libraries for this data. We thus use ICU for timezone data in preference to native APIs whenever ICU is configured (i.e. feature ICU is enabled; this at least requires sufficient ICU installed). The ICU backend for timezone data has recently had some changes that, it turns out (see QTBUG-129896), included some silly mistakes that weren't caught until after the 6.8.0 release, when a customer tried building with it. From this I infer that we have no MS-based platform configuration in Coin that tests with ICU enabled. Given that this is configuration is recommended (both by me and by MS), that is unfortunate.

Please adapt at least one Windows configuration to (provision ICU sufficiently and) configure to enable the use of ICU, so that we do catch future "fail to build from source" mistakes of this kind.

(We do also need to retain at least one Windows configuration that doesn't use ICU, so that we continue testing the legacy backend based on MS's registry-based access to timezone data.)