Add new package: stxxl

Here is a package for stxxl library (STL containers and tools for big datasets).
(needed this to compile OSRM on MinGW more conveniently than custom-made batch files, used in OSRM OpenStreetMap routing project for example)

This is my first PKGBUILD and I am not sure it is ready for inclusiion. Could someone have a look?

Always add at the end of the pkgdesc:
(mingw-w64)