The legacy `includedir` was removed in 4.0 (https://github.com/opencv/opencv/pull/12477) but it was not removed from the `.pc` file.
This causes issues when trying to link against OpenCV using pkg-config in CMake.
I didn't make the status part of the version variables here, because
letters don't play too well with version comparison algorithms
(especially with CMake's, which only supports numbers).