Fix 'tests' target build on macOS 15

[ihnorton]

On macOS 15, make tests is broken on current dev with errors like this:

pydev ❯ nj tests
[184/468] Building CXX object tiledb/stdx/test/CMakeFiles/unit_stdx.dir/compat.cc.o
FAILED: tiledb/stdx/test/CMakeFiles/unit_stdx.dir/compat.cc.o
/usr/bin/g++ -DNDEBUG -D_FILE_OFFSET_BITS=64 -I/Users/inorton/work/git/TileDB -isystem /Users/inorton/work/bld/TileDB-rel/vcpkg_installed/arm64-osx/include -O3 -DNDEBUG -std=c++20 -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.1.sdk -mmacosx-version-min=15.0 -fPIE -fvisibility=hidden -Wall -Wextra -Werror -O3 -Wno-unqualified-std-cast-call -MD -MT tiledb/stdx/test/CMakeFiles/unit_stdx.dir/compat.cc.o -MF tiledb/stdx/test/CMakeFiles/unit_stdx.dir/compat.cc.o.d -o tiledb/stdx/test/CMakeFiles/unit_stdx.dir/compat.cc.o -c /Users/inorton/work/git/TileDB/tiledb/stdx/test/compat.cc
In file included from /Users/inorton/work/git/TileDB/tiledb/stdx/test/compat.cc:34:
In file included from /Users/inorton/work/git/TileDB/tiledb/stdx/stop_token:41:
/Users/inorton/work/git/TileDB/tiledb/stdx/bits/stop_token.h:150:7: error: reference to '__stop_callback_base' is ambiguous
  150 |       __stop_callback_base* __cb,
      |       ^

gist: https://gist.github.com/ihnorton/2f616091f92be4b493f7fec6fdd40dee

It looks like AppleClang has added incomplete support for stop_token and std::jthread, but __cpp_lib_jthread guard doesn't exist, which breaks our existing feature guards in tiledb/stdx/{stop_token,thread}.

Fix is to add -fexperimental-library to AppleClang>16 cxx flags.

---

TYPE: NO_HISTORY
DESC: Fix 'tests' target build on AppleClang 16 (stop_token,jthread)

[teo-tsirpanis]

I think the root cause of the failures is that we are defining stuff in the std namespace, which is undefined behavior IIRC. We should use stdx or something else, and forward to std if the APIs are available on the platform.

[ihnorton]

We should use stdx or something else, and forward to std if the APIs are available on the platform.

The platform tests are broken on AppleClang 16 without this change because __cpp_lib_thread is not defined, so our conditional include for the alternative implementation is used.

[bekadavis9]

LGTM

[teo-tsirpanis]

(removing the requested changes, please create a story to change the std namespace for the polyfills)

[bekadavis9]

make tests still fails for me on macOS 15.1 after this fix.

A few errors include:

error: no member named 'copy' in namespace 'std::ranges'; did you mean 'bcopy'?

error: no member named 'permutable' in namespace 'std'

error: unexpected type name 'ZI': expected expression

error: no member named 'sortable' in namespace 'std'