unwind: fix symbol_name mem leack and some identations
properly compile code when libunwind is not present
use unsiged int to avoid float division (gcc bug thanks to Vlasenko)
major fixes to autoconf script

fix logic (don't fail on library not found)
change enable_libunwind with with_libunwind variable name
use check instead of maybe
unwind: adjusted several data type to use unsigned
fix for libunwind when using libpthread