Fix memset_s detection

Hi,

WIth mingw the memset_s detection is not severe enough:

conftest.c: In function 'main':
conftest.c:41:1: warning: implicit declaration of function 'memset_s' [-Wimplicit-function-declaration]
 (void) memset_s(dummy, (rsize_t) sizeof dummy, 0, (rsize_t) sizeof dummy);

I had to test the link to test memset_s availability.