Provide API for testing OpenSlide version

[bgilbert]

Consider exposing the OpenSlide version in a machine-readable form (perhaps with a comparison function or macro) so applications can apply version-specific workarounds without parsing the output of openslide_get_version(). Also consider how bindings will expose this information.

[bgilbert]

See #155 for an example use case.

[malaterre]

Since issue is still open, I'd like to extend the test to compile-time detection. Eg:

// only in 3.4.0 and up
#if OPENSLIDE_VERSION_MINOR > 3
  const char * vendor = openslide_detect_vendor( filename );
  return vendor == NULL;
#else
  return openslide_can_open( filename );
#endif

So OPENSLIDE_VERSION_MAJOR and OPENSLIDE_VERSION_MINOR would be exposed