The `-Wmost` flag was added by commit 97a51d2172 (ENH: add more xcode
stuff, 2005-01-27, v2.4.0~2396), along with a couple others, as part of
very early work on the Xcode generator. I suspect that the flags were
part of Xcode's project creation wizard. However, they are not
appropriate for CMake-based projects because we already have our own
mechanisms for specifying flags desired by the project or user.
Simply drop the flags.
Fixes: #13516
