setup.py: correctly handle None in library_dirs or include_dirs

If any of the entries in this list are None, cythonize (called in an
argument of setup at the end) will throw errors. The entries can be None
when the user has not set the environment variables for the MPI and FFTW
directories.

setup.py

@@ -107,14 +107,19 @@ def read(rel_path):
     else:
         raise RuntimeError("Unable to find version string.")
 
-# C-dependency paths for extension compilation and linking
-include_dirs = ['dedalus/libraries/fftw/',
+def filter_none(l):
+   return [d for d in l if d is not None]
+
+# C-dependency paths for extension compilation and linking. Note that cythonize will later throw errors if any of the entries below turn out to be None.
+include_dirs = filter_none([
+                'dedalus/libraries/fftw/',
                 np.get_include(),
                 mpi4py.get_include(),
                 get_include('mpi'),
-                get_include('fftw')]
+                get_include('fftw'),
+                ])
 libraries = ['fftw3_mpi', 'fftw3', 'm']
-library_dirs = [get_lib('fftw'), get_lib('mpi')]
+library_dirs = filter_none([get_lib('fftw'), get_lib('mpi')])
 
 # Warning supression
 extra_compile_args = ["-DNPY_NO_DEPRECATED_API=NPY_1_7_API_VERSION",