Define only least required macros for LFS in 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```. Also moved some complicated macro definitions from ```taglib.h``` to ```config.h``` and changed them to be defined when required.