Don't use __attribute__((__unused__)) on HP-UX w/o gcc.

popt/system.h

@@ -69,6 +69,10 @@ char *alloca ();
 #define	xstrdup(_str)	strdup(_str)
 #endif  /* HAVE_MCHECK_H && defined(__GNUC__) */
 
+#if defined(__GNUC__) || !(defined(__hpux) || defined(_hpux) || defined (hpux))
 #define UNUSED(x) x __attribute__((__unused__))
+#else
+#define UNUSED(x) x
+#endif
 
 #include "popt.h"