Automatically enable C11 atomics when available.

It's now 2021. Hopefully we can at least assume anyone building with
-std=c11 also has a corresponding set of headers. Plus, even if you
don't, Clang seems to provide a header. (So C11 atomics work in
clang-cl.) Also apparently atomics are optional, so this checks
__STDC_NO_ATOMICS__.

This does *not* set C11 as the minimum version. If you build with
-std=c99, we'll silently use the non-atomics implementation. That's a
little magical, so I've kept OPENSSL_C11_ATOMIC as a way to assert that
you really want C11 atomics. Mostly it turns into a -std=c11 && !MSVC
self-assert.

Update-Note: If something fails to compile, we'll revert this and adjust
the check, or add an opt-out, or give up. Also, if building with
-std=c99, consider -std=c11.

Change-Id: I1a8074c367a765c5a0f087db8c250e050df2dde8
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/46344
Reviewed-by: Adam Langley <agl@google.com>

crypto/internal.h

diff --git a/crypto/internal.h b/crypto/internal.h
index c574645..51d1d9c 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -471,6 +471,12 @@
 
 // Reference counting.
 
+// Automatically enable C11 atomics if implemented.
+#if !defined(OPENSSL_C11_ATOMIC) && !defined(__STDC_NO_ATOMICS__) && \
+    defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#define OPENSSL_C11_ATOMIC
+#endif
+
 // CRYPTO_REFCOUNT_MAX is the value at which the reference count saturates.
 #define CRYPTO_REFCOUNT_MAX 0xffffffff