### macOS: Drop the `-flat_namespace` linker flag

It's not clear when this flag was no longer required on macOS, we
tried dropping it
[in 2019](https://github.com/epics-base/epics-base/commit/cbb13bf6b170c6ba6660d594bdd9ca44a5bdaea5)
but had to [restore it](https://github.com/epics-base/epics-base/commit/8c3c5a9731887c6d8b1918394090564af38461b5).
XCode 15 seems a safe version though. See discussion in issue
[#895](https://github.com/epics-base/epics-base/issues/895) for
more details.