Enable building with exiv2-0.21 (#349150)

Due to API changes in exiv2-0.21 (see
http://dev.exiv2.org/issues/show/0000721

), gthumb with exiv2 support
enabled fails to compile. This patch updates gthumb to allow compilation
with both exiv2-0.20 and 0.21. Since ifd IDs are no longer part of the
public API, the patch uses group names instead (which works with
exiv2-0.20 as well, modulo the fact that 0.21 has isMakerGroup() instead
of isMakerIfd() in 0.20).
In addition, instead of numeric comparison with ids1Id (which wouldn't
work with exiv2-0.21 in any case), isMakerGroup()/isMakerIfd() are now
used to determine whether or not a tag is a MakerNote.

Signed-off-by: Alexandre Rostovtsev <tetromino@gmail.com>