add numpy again to the distribution include path

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.

setup.py

@@ -553,6 +553,13 @@ def main():
     exts = configure_special_extensions(exts, build_summary)
     exts = maybe_cythonize(exts)
 
+    try:
+        import numpy
+    except ImportError:
+        numpy_include_dirs = []
+    else:
+        numpy_include_dirs = [numpy.get_include()]
+
     classifiers = """\
         Development Status :: 5 - Production/Stable
         Intended Audience :: Science/Research
@@ -582,6 +589,7 @@ def main():
                     'build_ext': kwant_build_ext,
                     'build_tut': kwant_build_tut},
           ext_modules=exts,
+          include_dirs=numpy_include_dirs,
           setup_requires=['pytest-runner >= 2.7'],
           tests_require=['numpy > 1.6.1', 'pytest >= 2.6.3'],
           install_requires=['numpy > 1.6.1', 'scipy >= 0.11.0', 'tinyarray'],