Switch OPENSSL_COMPILE_ASSERT to static_assert in C++ code.

Clang for Windows does not like OPENSSL_COMPILE_ASSERT inside a function
in C++. It complains that the struct is unused. I think we worked around
this in C previously by making it expand to C11 _Static_assert when
available.

But libssl is now C++ and assumes a C++11-capable compiler. Use real
static_assert.

Bug: 132
Change-Id: I6aceb95360244bd2c80d194b80676483abb60519
Reviewed-on: https://boringssl-review.googlesource.com/17924
Reviewed-by: Adam Langley <agl@google.com>

ssl/ssl_lib.cc

diff --git a/ssl/ssl_lib.cc b/ssl/ssl_lib.cc
index 346f2c1..7441925 100644
--- a/ssl/ssl_lib.cc
+++ b/ssl/ssl_lib.cc
@@ -173,9 +173,9 @@
 
 /* Some error codes are special. Ensure the make_errors.go script never
  * regresses this. */
-OPENSSL_COMPILE_ASSERT(SSL_R_TLSV1_ALERT_NO_RENEGOTIATION ==
-                           SSL_AD_NO_RENEGOTIATION + SSL_AD_REASON_OFFSET,
-                       ssl_alert_reason_code_mismatch);
+static_assert(SSL_R_TLSV1_ALERT_NO_RENEGOTIATION ==
+                  SSL_AD_NO_RENEGOTIATION + SSL_AD_REASON_OFFSET,
+              "alert reason code mismatch");
 
 /* kMaxHandshakeSize is the maximum size, in bytes, of a handshake message. */
 static const size_t kMaxHandshakeSize = (1u << 24) - 1;
@@ -1083,7 +1083,7 @@
     return 0;
   }
 
-  OPENSSL_COMPILE_ASSERT(sizeof(cert->sid_ctx) < 256, sid_ctx_too_large);
+  static_assert(sizeof(cert->sid_ctx) < 256, "sid_ctx too large");
   cert->sid_ctx_length = (uint8_t)sid_ctx_len;
   OPENSSL_memcpy(cert->sid_ctx, sid_ctx, sid_ctx_len);
   return 1;