NF: generate config.pxi file with Cython DEF vars

Eleftherios needs to do conditional compiling depending on whether
openmp is present or not.

Following the recipe in
http://stackoverflow.com/questions/3826458/cython-conditional-compile-based-on-external-value
-- build a ``config.pxi`` file containing DEF variables that can be used
in extension code.  For example, with the code here, a Cython extension
could include the following:

    include "config.pxi"

    IF HAVE_OPENMP:
        cimport openmp
Merge pull request #445 from matthew-brett/conditional-compiling

NF: generate config.pxi file with Cython DEF vars