Currently, exiv2 installs in a somewhat nonsense way, if built with 'xmp' useflag.
It's an upstream shortcoming, yet still should be addressed.
xmp is built as a static lib, that exiv2 links to during build, then that static lib is still installed, but without any of its headers, making it somewhat redundant (unless something in the tree actually uses that static lib directly).
If nothing is the tree uses the static lib directly, is there a point of installing it, especially without its headers ?
This is when it got introduced: https://dev.exiv2.org/issues/0001119
Ideally should be fixed upstream.