Add -Wundef to --strict-warnings options.

Avoid a -Wundef warning in refcount.h
Avoid a -Wundef warning in o_str.c
Avoid a -Wundef warning in testutil.h
Include internal/cryptlib.h before openssl/stack.h
to avoid use of undefined symbol OPENSSL_API_COMPAT.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2712)