Silencing warnings for 'memcpy' macro redefined on macOS

[mauritium]

Silencing warnings for 'memcpy' macro redefined on macOS. Warnings would be like:

  CC       connection.lo
In file included from connection.c:3:
./lib.h:79:11: warning: 'memcpy' macro redefined [-Wmacro-redefined]
#  define memcpy(dest, src, n) i_memcpy(dest, src, n)
          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/secure/_string.h:62:9: note: previous definition is here
#define memcpy(dest, ...) \
        ^
1 warning generated.

This would massively pollute the build output on macOS.