Add the macro for the supported specification version

[akukanov]

Summary:

Implement the macro for the supported specification version, as described in uxlfoundation/oneAPI-spec#591:

// Defined in <oneapi/dpl/version>
#define ONEDPL_SPEC_VERSION /*implementation-defined*/

The ONEDPL_SPEC_VERSION macro must be defined to the decimal literal whose value equals to
major-spec-version * 100 + minor-spec-version, where major-spec-version and minor-spec-version
are the major and the minor versions of the latest fully supported specification.

The macro also needs to be documented in the developer guide.