Fix building against exiv2-0.27

One neeeded part is to copy over the new FindLibExiv2 module from
ECM which has not appeared in a release yet (it will with 5.53.0).
I'll probably add that version as a requirement and remove the copy
again in the near future.
The other part includes raising the cmake version requirement to
3.7.0, which was released over 2 years ago. (The version comparison
could be easily solved differently and less elegantly, but let's see
if people with ancient cmakes complain).