In other words, cut out poliqarp from build if this flag is False.
The flag remains forced to be True by default, though.
This is mostly to be able to cut poliqarp out of pure WIN32 build until
such time it actually builds (it doesn't atm).

Otherwise it will always try to build poliqarp/swig,
regardless of what you try to set in cmkae cache.
May the force be with you... NOT.

That should finally solve the issues of static vs dynamic libraries
conflicts that were arising when we tried to dynamically link boost
DLLs but didn't do anything about auto-linking feature that
wanted to link static libs.
ATM I left the default static linking on due to some warnings about
using non-DLL interfaced classes in classes that have DLL interfaces.

Leave defaults for linking Boost libraries on non-WIN32. WIN32 atm needs to link to static libraries for some reason.

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)

Libcorpus2 now contains the base strructures (tokens etc), their I/O and related classes.
Libpwrutils (name TBD) contains basic building blocks common to all tagger-related libraries
Tagset-tool only needs libcorpus, so was brough here as well.