Posix build (sort of) on Nixos

[Monocell]

Hi,

The install.md mentioned I should report working builds of wkhtmltopdf.

There is now build instructions for a qt-patched wkhtmltopdf for the Nixos distribution. It doesn't actually use the buildscript

in the repo since that assumes it has root access. I also had to patch the makefiles in two places to use 

$$INSTALLBASE

instead of

$(INSTALL_ROOT)

since Nixos is picky with install locations.

The build instructions can be viewed at https://github.com/NixOS/nixpkgs/tree/master/pkgs/tools/graphics/wkhtmltopdf

I'll try to answer any questions, but I'm basically just cargo culting so don't expect to much. :)

Thanks of a great tool.