Windows fixes

Minor release with mainly fixes to the Windows implib generation thanks to @dwbuiten.

Windows support and pkg-config customization

• Windows now works as intended thanks to @dwbuiten
• Added a --pkgconfigdir option (and fixed its default) thanks to @barrbrain
• prefix, libdir, includedir customization for the pkg-config now works thanks to @lu-zero

Initial release

They key features are implemented:

• cargo cbuild to build the libraries, header and pkg-config file
• cargo cinstall to install the whole lot