Avoid overlinking of OpenCV libs, clean up CMakeLists.txt a bit

[valgur]

Replaces

target_link_libraries(fbow ${OpenCV_LIBS})

with

target_link_libraries(fbow
    PUBLIC
        opencv_core
    PRIVATE
        opencv_features2d
        opencv_highgui
        "$<$<BOOL:${USE_CONTRIB}>:opencv_xfeatures2d>"
)

The linker otherwise pulls symbols from unrelated OpenCV libraries like opencv_dnn, which causes invalid shared library references to these to be added to the FBoW shared library.

I also bumped the CMake policy version to 3.12 to enable some modern CMake features and dropped C from the project languages since the project contains no C files.

Related to conan-io/conan-center-index#23889.

[ymd-stella]

LGTM