Support Exiv2 >= v0.28.0

The various Datum classes no longer have the toLong method and must be
replaced with toInt64.
ErrorCode is an enum class instead of an enum.
Error classes are reduced to Exiv2::Error.

rtengine/imageio.cc

@@ -86,6 +86,19 @@ FILE* g_fopen_withBinaryAndLock(const Glib::ustring& fname)
     return f;
 }
 
+template <typename Iterator, typename Integer = std::size_t>
+auto to_long(const Iterator &iter, Integer n = Integer{0}) -> decltype(
+#if EXIV2_TEST_VERSION(0,28,0)
+    iter->toInt64()
+) {
+    return iter->toInt64(n);
+#else
+    iter->toLong()
+) {
+    return iter->toLong(n);
+#endif
+}
+
 }
 
 void ImageIO::setMetadata(Exiv2Metadata info)
@@ -1185,7 +1198,7 @@ int ImageIO::saveTIFF (
         }
         it = exif.findKey(Exiv2::ExifKey("Exif.Image.ResolutionUnit"));
         if (it != exif.end()) {
-            res_unit = it->toLong();
+            res_unit = to_long(it);
         }
     }
     TIFFSetField(out, TIFFTAG_XRESOLUTION, x_res);