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>
diff --git a/ssl/handshake_client.cc b/ssl/handshake_client.cc
index dfb9c92..9efbf0a 100644
--- a/ssl/handshake_client.cc
+++ b/ssl/handshake_client.cc
@@ -1512,8 +1512,8 @@
   return 1;
 }
 
-OPENSSL_COMPILE_ASSERT(sizeof(size_t) >= sizeof(unsigned),
-                       SIZE_T_IS_SMALLER_THAN_UNSIGNED);
+static_assert(sizeof(size_t) >= sizeof(unsigned),
+              "size_t is smaller than unsigned");
 
 static int ssl3_send_client_key_exchange(SSL_HANDSHAKE *hs) {
   SSL *const ssl = hs->ssl;