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/base.h

diff --git a/include/openssl/base.h b/include/openssl/base.h
index 47b3ccb..65ab3f7 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -138,6 +138,15 @@
 
 #endif  /* defined(BORINGSSL_SHARED_LIBRARY) */
 
+
+#if defined(__GNUC__)
+#define OPENSSL_PRINTF_FORMAT_FUNC(string_index, first_to_check) \
+        __attribute__((format(printf, string_index, first_to_check)))
+#else
+#define OPENSSL_PRINTF_FORMAT_FUNC(string_index, first_to_check)
+#endif
+
+
 /* CRYPTO_THREADID is a dummy value. */
 typedef int CRYPTO_THREADID;