Enable __STDC_WANT_LIB_EXT1__ to get memset_s

As we switch to C11, the availability of the memset_s function under Macos is
detected by autoconf. The explicit_bzero.c module will use it to implement
explicit_bzero(3) on that system.

According to N1570, Annex K, in order for the string.h header to declare
memset_s(), we need to define __STDC_WANT_LIB_EXT1__ as 1 before including the
header file.  The libc implementation is in turn supposed to define
__STDC_LIB_EXT1__ in the header, to claim that the interfaces of Annex K are
implemented as dictated by the standard.

In theory, we should be using memset_s() only if __STDC_LIB_EXT1__ is defined,
but we know that memset_s() exists thanks to autoconf. Since the Macos libc
does not define __STDC_LIB_EXT1__, we can not be 100% sure that the memset_s()
function is implemented in a standard way.

explicit_bzero.c

@@ -5,6 +5,7 @@
  * Written by Ted Unangst
  */
 
+#define __STDC_WANT_LIB_EXT1__ 1
 #include <string.h>
 
 /*