-O2 and "no -O2" needed

[cron2]

During work on openvpn#836 it was found that the mingw buildbot compiles with -O2 (as it should) and the GHA mingw/cmake build seems to not use -O2, which makes a notable difference for static inline foo() vs. non-static inline foo() definitions - the latter fail to link (not inlined, and not instantiated) when compiling without optimization.

Can we add a mingw build with no -O2 for that? Or any other good suggestion how to catch such omissions?