It has the same problems as setup_requires.

Packages that are mentionned there are installed in a way that bypasses
pip:
https://pip.pypa.io/en/stable/user_guide/?highlight=setup_requires#installation-bundles

These prefixes were visible, for example when invoking
"./setup.py --help build".

+ References to numpy arrays are removed from docstrings, instead
  general sequences are accepted

+ `__call__` no longer returns `where`

+ a `sum` attribute was added (and the corresponding arg in __init__. If
  sum==True then __call__ evaluates ∑_{iαβ} φ*_α Q_{iαβ} ψ_β. If
  sum==False then __call__ evaluates ∑_{αβ} φ*_α Q_{iαβ} ψ_β (i.e. no sum
  over the site/hopping index i.

Before we used the include_dir argument to setup(), but this caused problems:
kwant/kwant#48

Commit a8b0463e turned out to be a bad
idea: unlike on Debian, on many platforms the numpy headers are not
available in the default include path.

    1. Now can be compiled without MUMPS
       (previously exception was thrown)
    2. Gives sensible error message for Git install,
       but without installed Cython.

With the reorganization of setup.py, most constants are now used in a
single place.  It is clearer and simpler to have them there.

The CYTHON_TRACE macro can be now set in the build.conf file.

All C extensions are now treated on equal footing and can be configured
in "build.conf".  All the options known by Extension may be set now for
any extension individually and for all of them together (using the
[DEFAULT] section).

This reduces the number of global variables and makes the code easier to
understand.