BDE does not compile on current macOS

**Describe the bug**
On the compilers that come with Xcode command line tools 26.6, build fails with:

```console
/usr/bin/c++ -DBDE_BUILD_TARGET_CPP17 -DBDE_BUILD_TARGET_DBG -I/Users/builduser/Data/b/bde/src/4.38.0.0-3c141dc2f3.clean/groups/bsl/bsls -I/Users/builduser/Data/b/bde/src/4.38.0.0-3c141dc2f3.clean/thirdparty/bbryu/.. -fPIC -g -std=c++17 -arch arm64 -MD -MT groups/bsl/CMakeFiles/bsls-iface.dir/bsls/bsls_compilerfeatures.cpp.o -MF groups/bsl/CMakeFiles/bsls-iface.dir/bsls/bsls_compilerfeatures.cpp.o.d -o groups/bsl/CMakeFiles/bsls-iface.dir/bsls/bsls_compilerfeatures.cpp.o -c /Users/builduser/Data/b/bde/src/4.38.0.0-3c141dc2f3.clean/groups/bsl/bsls/bsls_compilerfeatures.cpp
/Users/builduser/Data/b/bde/src/4.38.0.0-3c141dc2f3.clean/groups/bsl/bsls/bsls_compilerfeatures.cpp:454:7: error: __cpp_sized_deallocation is not defined
454 | # error __cpp_sized_deallocation is not defined
| ^
1 error generated.
```

when built in vcpkg.

**To Reproduce**
1. Get a macOS 26.6 / XCode 26.6 machine
2. Clone https://github.com/microsoft/vcpkg
3. ./bootstrap-vcpkg.sh
4. `vcpkg install bde`

**Expected behavior**
Passing build.

**Screenshots**
N/A

**Environment (please complete the following information):**
- Operating System and Version: macOS 26.6.
- Browser [e.g. Firefox, Safari] (if applicable): N/A

**Additional context**

This problem was detected trying to update the vcpkg build lab in https://github.com/microsoft/vcpkg/pull/53316

Reproduce the failure with vcpkg on macOS 26.6 using Xcode command line tools 26.6, then inspect groups/bsl/bsls/bsls_compilerfeatures.cpp around line 454 and the compiler-feature checks shown in the error. Done means the BDE package builds successfully through `vcpkg install bde` on the reported environment.

Written by the indexing model from the issue text.