Don't enable atomics in NO_THREADS configurations.

In configurations without threads, we're not thread-safe anyway. Instead
use the refcount_lock.c implementation which, in turn, calls into
thread_none.c, so this turns into a plain refcount.

This avoids a build issue on platforms which define NO_THREADS, use C11,
lack C11 atomics, and are missing a __STDC_NO_ATOMICS__ definition. The
platforms ought to define __STDC_NO_ATOMICS__ or implement them, but
atomics are also unnecessary overheard in NO_THREADS configurations
anyway.

Change-Id: I927e1825dd6474d95226b93dad704594f120450a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/48565
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/internal.h

diff --git a/crypto/internal.h b/crypto/internal.h
index b288583..03bb779 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -492,8 +492,9 @@
 // Reference counting.
 
 // Automatically enable C11 atomics if implemented.
-#if !defined(OPENSSL_C11_ATOMIC) && !defined(__STDC_NO_ATOMICS__) && \
-    defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#if !defined(OPENSSL_C11_ATOMIC) && defined(OPENSSL_THREADS) &&   \
+    !defined(__STDC_NO_ATOMICS__) && defined(__STDC_VERSION__) && \
+    __STDC_VERSION__ >= 201112L
 #define OPENSSL_C11_ATOMIC
 #endif