Check for SecureZeroMemory at compile time.

configure.ac

@@ -148,6 +148,10 @@ dnl Checks for typedefs, structures, and compiler characteristics.
 
 AC_C_BIGENDIAN
 
+dnl Checks for functions and headers
+
+AC_CHECK_FUNCS([SecureZeroMemory])
+
 dnl Switches.
 
 AC_ARG_ENABLE(pie,

src/libsodium/randombytes/randombytes_salsa20_random.c

@@ -51,7 +51,7 @@ static Salsa20Random stream = {
 static void
 sodium_memzero(void * const pnt, const size_t size)
 {
-#ifdef _WIN32
+#ifdef HAVE_SECUREZEROMEMORY
     SecureZeroMemory(pnt, size);
 #else
     volatile unsigned char *pnt_ = (volatile unsigned char *) pnt;