Use MallocAllocator for macOS.
Use MallocAllocator for macOS.

Specifically, force include magic/malloc_allocator.hpp on macOS
builds via clang argument.

pleroy/Principia,mockingbirdnest/Principia,mockingbirdnest/Principia,mockingbirdnest/Principia,pleroy/Principia,pleroy/Principia,pleroy/Principia,mockingbirdnest/Principia