After upgrading the macOS system dev tools, building Emacs fails because aligned_alloc is present in the system headers and can be linked with but isn't actually in the dynamic library (macOS 10.14, Xcode 11).

This means that the configure check for that function needs to be strengthened a bit. Proposed patch attached; please say if it breaks on a different platform. I'm not the greatest fan of autoconf, and am likely to have made mistakes.