When building taglib:
    cerbero build taglib
error:
    CMake Error: No source or binary directory provided

This is because no build directory specified, so add '.' for it.
Signed-off-by: dudengke <pinganddu90@gmail.com>

This throws an error when running with CMake 3.13.3, and is a warning
for later versions.
Signed-off-by: Nirbheek Chauhan <nirbheek@centricular.com>