Fix compile bug for Mac OS X 10.13.6
Master branch does not compile on some Unix systems since commit d077f9a8154df309b1b715e51d663e4cd6cf8e73 (for instance compilation fails on Mac OS X 10.13.6 using GNUC 10.4.2).
The problem is that on such systems CMSG_SPACE() is not a compile time constant, so it cannot be used to allocate the size of the message buffer we use in shm_unix.h
The fix is to use a dynamic array instead of a stack array for the buffer. We use a standard C++ vector to get automatic memory management, and align it with our utilities in memory.h.
fixes https://github.com/official-stockfish/Stockfish/issues/7092
closes https://github.com/official-stockfish/Stockfish/pull/7093
No functional change