ECDSA support

    Support for ECDSA is added for use with OpenSSL >= 1.0.2.
    OpenSSL had an outstanding bug report, #2459, that requests
    the structure ecdsa_method be exposed. An engine needs to create
    a modified version of the structure, which has pointers into
    functions within the engine.  Code as suggested in #2459 was
    added in OpenSSL master and in OpenSSL-1.0.2-beta.

    Engines using RSA do not have this problem, because the
    rsa_method_st is exposed in rsa.h This allows an engine such
    as the combination of libp11 and engine_pkcs11 to compile in
    a static version of the rsa_method_st.

    These libp11 modifications are designed to allow building
    libp11 using either the ESDA_METHOD_new or (for backward
    compatibility with older OpenSSL versions) use the internal OpenSSL
    crypto/ecdsa/ecs_locl.h

    By default the ECDSA_METHOD_new will be used if present.

    To build using the ecs_locl.h, one must have access to the
    OpenSSL source and add to the libp11 build process,
    -DBUILD_WITH_ECS_LOCL_H -I/<path.to.OpenSSL.source>/crypto/edcsa

    (Note: that using an internal header file may require libp11
    to be rebuilt to match the specific version of OpenSSL being
    used, and is not recommened.)