Add memmove_s emu

Some applications, like CryptoPP, won't run on Windows XP, because
memmove_s isn't defined in that OS. As I needed CryptoPP to compile
easily, I wrote this patch. It emulates the method by checking for the
availability and falling back on memmove (without s).

Patch by Ruben de Smet ruben.de.smet a telenet.be