use the msvc-compatible string() casts instead of the working on gcc, but probably noncompliant, char* to itertor casts

Looks like there's no conversion between const char* and string::const_iterator on MSVC (actually it sounds safer if you ask me). Anyway, working around that by wrapping const char* around a std::string

If SIGUSR1 is not defined, skip the code that registers handler for TokenTimer (so yes, it is not fully functional on a system without SIGUSR1).

Make add_library use default setting which is set to SHARED libs for anything but WIN32. WIN32 libs are going to be STATIC until external API is specifically defined (Windows' DLLs export nothing by default, UNIX shared libs export everything)

implement get_pos_value_mask and get_pos_required_mask
use int for *_count to avoid signed/unsigned errors
handel missing required attributes partially, add a todo

move count_bits_set to pwrutisl, put lowest_bit alias there, implement Tagset::tag_size and Tagset::tag_is_singular