Add GCC attributes when compiled with gcc. This helps find out if

we're using the printing functions correctly or not.

I used the corresponding attributes found in the header files of my
Linux installation.

crypto/bio/bio.h

@@ -612,10 +612,17 @@ void BIO_copy_next_retry(BIO *b);
 
 /*long BIO_ghbn_ctrl(int cmd,int iarg,char *parg);*/
 
-int BIO_printf(BIO *bio, const char *format, ...);
-int BIO_vprintf(BIO *bio, const char *format, va_list args);
-int BIO_snprintf(char *buf, size_t n, const char *format, ...);
-int BIO_vsnprintf(char *buf, size_t n, const char *format, va_list args);
+#ifndef __GNUC__
+#define __attribute__(x)
+#endif
+int BIO_printf(BIO *bio, const char *format, ...)
+	__attribute__((__format__(__printf__,2,3)));
+int BIO_vprintf(BIO *bio, const char *format, va_list args)
+	__attribute__((__format__(__printf__,2,0)));
+int BIO_snprintf(char *buf, size_t n, const char *format, ...)
+	__attribute__((__format__(__printf__,3,4)));
+int BIO_vsnprintf(char *buf, size_t n, const char *format, va_list args)
+	__attribute__((__format__(__printf__,3,0)));
 
 /* BEGIN ERROR CODES */
 /* The following lines are auto generated by the script mkerr.pl. Any changes