I try to cleanly separate between information that should be part of the
Kwant source distribution (that is tied to a particular version of
Kwant), and generic information that is best presented on the website.

The pip tool messes up the timestamps of files, so with our previous strategy
of enabling cython by default the build fails if Cython is not available.

Also, turn the "warning" into a "caution" message that is less frightening and
send it to stdout only, so that it does not appear with pip.

This offers better dependency handling (the compilation of some .c files
depends on build.conf, but their cythonization doesn't), and other advantages
like parallel builds.

We do not use it, but this makes sure that it's included in SOURCES.txt.  This
is necessary so that source distributions can be recreated identically from
source distributions.

When the --no-cython option is given, it is no longer an error when
derived files are older than source files.

Error messages have been improved.

The file kwant/_static_version.py is no longer written upon every
invocation of setup.py.  Instead, a file kwant/_kwant_version.py is
injected into source and binary distributions.

The static version in kwant/_kwant_version.py now takes precedence
before any version from git.  This makes the --no-git option to setup.py
no longer necessary, since the existence of kwant/_kwant_version.py has
the same role.  The motivation behind this change is to allow keeping
Kwant distributions in git repositories that have a different layout
from the canonical Kwant git repository.  This is useful for example for
packaging.

The name of the static version file was changed to avoid problems with
stale files.