compat: Fix __has_attribute usage

Fixes: #195
Link: #195

Author: aabc

compat.h

@@ -774,7 +774,7 @@ struct module *find_module(const char *name)
 /* Copy from 294f69e662d1 ("compiler_attributes.h: Add 'fallthrough' pseudo
  * keyword for switch/case use") */
 #ifndef fallthrough
-# if __has_attribute(__fallthrough__)
+# if defined __has_attribute && __has_attribute(__fallthrough__)
 #  define fallthrough			__attribute__((__fallthrough__))
 # else
 #  define fallthrough			do {} while (0)  /* fallthrough */