Protect C language extensions with two leading and trailing underscores

This should make their usage safer against user macros.

src/explicit_bzero.c

@@ -6,7 +6,7 @@
 
 #include <string.h>
 
-__attribute__((weak)) void
+__attribute__((__weak__)) void
 __explicit_bzero_hook(void *buf, size_t len)
 {
 }