Fix opencv build system

In any case please apply the first thunk, typo precluding compilation of libflandmark without additional include paths. Keep in mind that CMake variables are case-sensitive. There are in fact 2 typos, s#OPENCV#OpenCV# s#DIR#DIRS#.

The second thunk ensures that all dependent objects get linked in; on Windows, OpenCV has a static 3rdparty zlib dependency which breaks shared build. This thunk may be dubious however as it's enlarging shared binary without -Wl,-as-needed or -flto. Gets rid of platform-specific stuff however.