Adds opt-in library compilation support and C-linking

By specifying `-DWITH_LIBRARY=YES` as a cmake flag, one opts in to compile a library (`libblp`) allowing low-level access to BLP width, height and pixel data without having to rely on the command-line binary.

C-linkage is added to ensure that `libblp` can be used from C, but also languages such as Ruby through foreign interfaces.

Many thanks for BLPConverter! I'm currently using this pull request approach happily in [Blizzardry](https://github.com/timkurvers/blizzardry).