<malloc.h>: Restrict support for <malloc.h> to !defined(__STDC__).

<malloc.h>: Restrict support for <malloc.h> to !defined(__STDC__).

In essence this is what FreeBSD did: error if __STDC__ is defined, and
silently include <stdlib.h> if not.

Packages are expected to now fail their config checks for <malloc.h>
but to build nevertheless, which was confirmed with building ~500
packages as a test.

Adjust a few config.h files of contrib/ code as well, notably
libssp's, which gets rid of the malloc.h warnings from the buildworld
output.