build: test for timingsafe_bcmp

Code introduced in #15649 added usage of `timingsafe_bcmp()`, if
available, otherwise falling back to our own implementation. However
the relevant build system check was never added, so currently, we'll
always just use our implementation, as HAVE_TIMINGSAFE_BCMP will never
be defined.

Add the check for timingsafe_bcmp. Note that as far as I'm aware, it's
only available on OpenBSD.

configure.ac

@@ -998,6 +998,8 @@ AC_CHECK_DECLS([setsid])
 
 AC_CHECK_DECLS([pipe2])
 
+AC_CHECK_FUNCS([timingsafe_bcmp])
+
 AC_CHECK_DECLS([le16toh, le32toh, le64toh, htole16, htole32, htole64, be16toh, be32toh, be64toh, htobe16, htobe32, htobe64],,,
     [#if HAVE_ENDIAN_H
                  #include <endian.h>

src/crypto/chacha_poly_aead.cpp

@@ -2,6 +2,10 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#if defined(HAVE_CONFIG_H)
+#include <config/bitcoin-config.h>
+#endif
+
 #include <crypto/chacha_poly_aead.h>
 
 #include <crypto/poly1305.h>