Updated the install process using SCons

It's possible to choose a different PREFIX using arguments

    scons -Q PREFIX=/usr/local
    scons install

Using scons make it easier to update, I tried making a standard autoconf/automake setup but gave up after 3 hours to make it build correctly with scons in less than a hour. The reason why I updated the build process is to make it easier to integrate with "Homebrew" for example. I'm trying to add libsoil to homebrew.