Spell __attribute__((format(printf, ...))) with more underscores.

They may be spelled with or without underscores. Alas, a lot of C code (adb,
cURL) seems to find it a popular pasttime to #define printf *before* including
external headers. This is completely nonsense and invalid, but working around
it is easy and is what we (and OpenSSL) were doing before
061332f21643a910941c32cbfc40e577c380e342.

I'll be sending a patch to cURL tomorrow to make them at least do their macro
trickery after external #includes for sanity. adb's sysdeps.h is a lot longer
and consistently #included first so I'll probably leave that be for lack of
time.

Change-Id: I03a0a253f2c902eb45f45faace1e5c5df4335ebf
Reviewed-on: https://boringssl-review.googlesource.com/7605
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/base.h

diff --git a/include/openssl/base.h b/include/openssl/base.h
index 0379b74..a41e13b 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -142,7 +142,7 @@
 
 #if defined(__GNUC__)
 #define OPENSSL_PRINTF_FORMAT_FUNC(string_index, first_to_check) \
-        __attribute__((format(printf, 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