Partially revert "check_snmp: Close potential for using uninitialized memory"

This reverts the changes to plugins/common.h applied by commit
a20611d4357c3c3ebe0a62776a1642e7904c1410.

- Clang defines __GNUC__.
- The new name of the __attribute__ parameter was misleading.

plugins/common.h

@@ -208,9 +208,9 @@ enum {
 # define bindtextdomain(Domainname, Dirname) /* empty */
 #endif
 
-/* For non-GNU/non-clang compilers to ignore __attribute__ */
-#if !defined(__GNUC__) && !defined(__CLANG__)
-# define __attribute__(noreturn) /* do nothing */
+/* For non-GNU compilers to ignore __attribute__ */
+#ifndef __GNUC__
+# define __attribute__(x) /* do nothing */
 #endif
 
 #endif /* _COMMON_H_ */