fix "unresolved external symbol round" under windows

Include round via igraph_math.h
python: special fallback includes on windows

On the windows the current fallbacks in setup.py do not make sense.
Include the dirs as if we run setup from the git checkout source, but
*only* if there is *one* such directory. Add a hint if there are two
or the msvc build was not build...
Add a windows-python target and script

The script can be used to build a python packages from cygwin
1) clone igraph
2) in cygwin: ./bootstrap.sh; ./configure;
3) make windows-python

-> will build the igraph-*-msvc direcory, build it via msvc tools (must be
available!) and then the python bindings. The makefile uses the script
tools/build_python_windows.cmd