Meson build support
This merge request adds Meson build support to libdvdcss. As the library already used -fvisibility=hidden there was no change to the exported symbols. I've tested the build on Windows (MinGW and MSVC), Linux and macOS. Other operating systems such as FreeBSD and Solaris don't currently compile though, since there have been some recent additions and the code for other OSes hasn't been touched in a long time. The build system should work on them though, in case anyone wants to fix the code.