Use OPENSSL_UNUSED in OPENSSL_COMPILE_ASSERT.

This would also have fixed the Windows clang issues. Those kicked in
because Windows clang defines __clang__ and not __GNUC__, but
OPENSSL_UNUSED accounts for this. It's also shorter.

Change-Id: I75bc17bbb789c5b78a7a369c43194e146739f574
Reviewed-on: https://boringssl-review.googlesource.com/18004
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
diff --git a/include/openssl/type_check.h b/include/openssl/type_check.h
index 7e70918..a6f8284 100644
--- a/include/openssl/type_check.h
+++ b/include/openssl/type_check.h
@@ -78,13 +78,9 @@
 
 #if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
 #define OPENSSL_COMPILE_ASSERT(cond, msg) _Static_assert(cond, #msg)
-#elif defined(__GNUC__)
-#define OPENSSL_COMPILE_ASSERT(cond, msg)                      \
-  typedef char OPENSSL_COMPILE_ASSERT_##msg[((cond) ? 1 : -1)] \
-      __attribute__((unused))
 #else
 #define OPENSSL_COMPILE_ASSERT(cond, msg) \
-  typedef char OPENSSL_COMPILE_ASSERT_##msg[((cond) ? 1 : -1)]
+  typedef char OPENSSL_COMPILE_ASSERT_##msg[((cond) ? 1 : -1)] OPENSSL_UNUSED
 #endif