Update graphics/exiv2 to 0.25.

As there is an shlib version bump, bump them portrevision of dependent ports.

While doing so, also switch to the cmake build system, as it requires less
patching and is easier to handle.

PR: 211329
Reviewed by: mat, rakuco, kwm
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D7283