libmpdec/memory.c keeps pointers to customisable memory functions (malloc/free) and initialises them to &malloc and &free. These functions are dllimport'd from the CRT, and so they trigger a warning letting you know that "&malloc == &malloc" may not always be true.

(That trivial comparison should always be true, but if you indirect one of the values through a few other modules it may be an address of a different stub function that calls the real malloc, and hence the addresses may not match.)