Update Windows build instructions

[axel-habermaier]

While building LtsMin on Windows for use with our tool, I found the following to be necessary; I have no idea if there is an easier way to achieve this.

1. Cygwin, by default, does not export any symbols in the resulting .exe files, which is required in order for pins2lts-seq, pins2lts-mc and pins2lts-sym to work with our C++ plugin. To fix this, run the following command and then rebuild LtsMin:
   ./configure --disable-dependency-tracking LDFLAGS=-Wl,--export-all-symbols
2. In order to be able to link against the executables using Visual Studio, the appropriate .lib files have to be generated; this step might be optional if you dynamically load the required entry points. I'll look into this at some point, but it appears to be rather inconvenient if not particularly hard to do so. What you can do instead is also annoying, but at least you only have to do it once for each executable: Follow the steps outlined here: https://adrianhenke.wordpress.com/2008/12/05/create-lib-file-from-dll/
   Update: So far, it seems that this step is only necessary after the publicly exported symbols have changed (and with the above, all functions and global variables seem to be exported). So adding a new function to LtsMin requires new .lib files, while changing the implementation of a function does not.

Also, note how "Cygwin is still only 32 bit" in your readme is outdated; LtsMin works fine on cygwin x64 on Windows 8 and 10.

[alaarman]

Removed the "Cygwin is still only 32 bit" from the README

Not sure about how to export the symbols and VS.