Define API version macros for other code to query

Note the distinction between the version (1.0) and the release label
(2013.03). The latter is not really usable to check for API
compatibility. (See <http://www.semver.org>).

opm/core/version.h

@@ -2,3 +2,10 @@
  * This symbol is initialized with the built version of the library.
  */
 extern const char* const opm_core_version;
+
+/**
+ * Current API level (for use with DUNE_VERSION_xxx):
+ */
+#define OPM_CORE_MAJOR    1
+#define OPM_CORE_MINOR    0
+#define OPM_CORE_REVISION 0