GCC doesn't care, but Clang is strict about this.

Change-Id: If34305303a87459985c98423a8283cad704ae984

6e1f6456 tried to do this, but MSVC doesn't short-circuit #if
statements. So this change tries having the test be in a different #if.

Change-Id: Id0074770c166a2b7cd9ba2c8cd06245a68b77af8

OpenSSL has traditionally done reference counting with |int|s and the
|CRYPTO_add| function. Unless a special callback is installed (rare),
this is implemented by doing the reference count operations under a
lock.

This change adds infrastructure for handling reference counts and uses
atomic operations when C11 support is available.

Change-Id: Ia023ce432319efd00f77a7340da27d16ee4b63c3
Reviewed-on: https://boringssl-review.googlesource.com/4771
Reviewed-by: Adam Langley <agl@google.com>