depends: fix osx build with clang 16
For some reason the previous syntax worked with clang 15 and below, but
clang 16 requires that the option and value are properly separated.
depends: remove redundant stdlib option
Use of -stdlib++-isystem gets rid of any system c++ header include paths and
negates the need for this option. In newer versions of clangs the combo
produces a warning.
depends: modernize clang flags
Fixes builds with llvm >= 11 in guix by working around the problem. As a bonus,
this is much cleaner and more maintainable than what we had before.
This is a backport of core#27328, core#27721 and core#27798
The change to ci/test/00_setup_env_mac.sh in core#27798 to ignore -Wunused-command-line-argument warnings seems equivalent to D3792