Use the meta-package x86_64-microarch-level in requirements/build in conda recipes to set up the compiler flags and set up the virtual package requirements in the run requirements.

When building packages on CI, level=4 will not be guaranteed, so you can only use level<=3 to build.

The run_exports only has a lower bound and therefore a level=2 build can be installed on a level=3 user system. A tighter bound is not added because we want to be able to test both level=2 and level=3 on a CI machine with level=3. Therefore in order to prioritise the highest level, use the build number to prioritise the level.

Only supported on Linux and macOS.