Only define _CRT_RAND_S when not already defined.

Only define _CRT_RAND_S when not already defined.

There exist builds which go out of their way to define _CRT_RAND_S for everything by forcing it into the list of macros defined on the command line to the compiler. Unfortunately, something like "-D_CRT_RAND_S" implies _CRT_RAND_S is defined to be '1' and defining it to be empty produces a redefinition.

Since the intention here is to ensure that _CRT_RAND_S is always defined, only define it if it isn't already defined.