C++17 deprecated the two-argument version of std::allocator::allocate.
Starting with 11.0, libc++ enforces these deprecations, which causes
warnings from the Boost headers, and since we require native Linux
builds to be warning-free, this breaks CI.
Boost addressed this issue for MSVC in 1.75; the patch in this commit
forces Clang to be handled in the same way.
Jack Grigg