Introduce libsiesta for easier use with SiestaAsASubroutine programs

I use the SiestaAsASubroutine functionality of Siesta when building PSiesta. Unfortunately, finding and linking all the right parts of Siesta is kind of hard. This MR introduces a "libsiesta.a" which is compiled as a by-product of the normal build process. It reduces the complexity of linking at least a little bit. It could perhaps also become simpler to build some of the utils, but I haven't looked into that.

It also opens the possibility of installing libsiesta to a "system" placement for easier compilation of user SiestaAsASubroutine programs. In that case the module files would also need to be installed to an include-dir. And if we then added a pkgconfig file -- then linking would be super easy :) I haven't added any install (or pkgconfig) functionality in this MR though - I think that is best left for another MR - but it's just to mention a perspective that I think is nice.

Let me know what you think -- i would much prefer for something like this to be in the main repo rather than in a branch of my own.