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)
 {
 }

src/local-link.h

@@ -29,5 +29,5 @@
 
 #define libbsd_link_warning(symbol, msg) \
 	static const char libbsd_emit_link_warning_##symbol[] \
-		__attribute__((used,section(".gnu.warning." #symbol))) = msg;
+		__attribute__((__used__,__section__(".gnu.warning." #symbol))) = msg;
 #endif

src/setproctitle.c

@@ -287,9 +287,12 @@ __asm__(".symver setproctitle_impl,setproctitle@@LIBBSD_0.5");
  * for code linking against that version, and change the default to use the
  * new version, so that new code depends on the implemented version. */
 #ifdef HAVE_TYPEOF
-extern typeof(setproctitle_impl) setproctitle_stub __attribute__((alias("setproctitle_impl")));
+extern __typeof__(setproctitle_impl)
+setproctitle_stub
+	__attribute__((__alias__("setproctitle_impl")));
 #else
-void setproctitle_stub(const char *fmt, ...)
-	__attribute__((alias("setproctitle_impl")));
+void
+setproctitle_stub(const char *fmt, ...)
+	__attribute__((__alias__("setproctitle_impl")));
 #endif
 __asm__(".symver setproctitle_stub,setproctitle@LIBBSD_0.2");

src/setproctitle_ctor.c

@@ -49,4 +49,4 @@
  * move them from .ctors to .init_array.
  */
 void (*libbsd_init_func)(int argc, char *argv[], char *envp[])
-	__attribute__((section(".init_array"))) = setproctitle_init;
+	__attribute__((__section__(".init_array"))) = setproctitle_init;