Added the CMake check for LFS and moved macros to config.h.

While investigating the issue #411, I found that current ```TagLib2``` branch doesn't work on some systems (e.g. PowerPC version of Ubuntu) due to the wrong definition of ```offset_t```. 
So I fixed the definition of ```offset_t```. And moved some macros from ```taglib.h``` to ```config.h``` and changed to define the least required ones.