Define |OPENSSL_PRINTF_FORMAT_FUNC| for format string annotations.

This centralizes the conditional logic into openssl/base.h so that it
doesn't have to be repeated. The name |OPENSSL_PRINTF_FORMAT_FUNC| was
chosen in anticipation of eventually defining an
|OPENSSL_PRINTF_FORMAT_ARG| for MSVC-style parameter annotations.

Change-Id: I273e6eddd209e696dc9f82099008c35b6d477cdb
Reviewed-on: https://boringssl-review.googlesource.com/6909
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/bio.h

diff --git a/include/openssl/bio.h b/include/openssl/bio.h
index c88a3e1..0c8bfe4 100644
--- a/include/openssl/bio.h
+++ b/include/openssl/bio.h
@@ -312,14 +312,8 @@
  *
  * These functions are versions of printf functions that output to a BIO rather
  * than a FILE. */
-#ifdef __GNUC__
-#define __bio_h__attr__ __attribute__
-#else
-#define __bio_h__attr__(x)
-#endif
 OPENSSL_EXPORT int BIO_printf(BIO *bio, const char *format, ...)
-    __bio_h__attr__((__format__(__printf__, 2, 3)));
-#undef __bio_h__attr__
+    OPENSSL_PRINTF_FORMAT_FUNC(2, 3);
 
 
 /* Utility functions. */