pyoperators: FTBFS: NameError: global name 'FFTW_DEFAULT_NUM_THREADS' is not defined

Imported from Debian bug http://bugs.debian.org/805678:

Source: pyoperators
Version: 0.13.6-1
Severity: serious
Justification: fails to build from source
Tags: sid stretch
User: <email address hidden>
Usertags: ftbfs
X-Debbugs-CC: <email address hidden>

Dear Maintainer,

The package fails to build:

======================================================================
ERROR: Failure: ValueError (Attempted relative import in non-package)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/nose/loader.py", line 420, in loadTestsFromName
    addr.filename, addr.module)
  File "/usr/lib/python2.7/dist-packages/nose/importer.py", line 47, in importFromPath
    return self.importFromDir(dir_path, fqname)
  File "/usr/lib/python2.7/dist-packages/nose/importer.py", line 94, in importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File "/pyoperators-0.13.6/.pybuild/pythonX.Y_2.7/build/test/test_core.py", line 30, in <module>
    from .common import OPS, ALL_OPS, DTYPES, HomothetyOutplaceOperator
ValueError: Attempted relative import in non-package

======================================================================
ERROR: test_fft.test_convolution_real(array([[ 0., 0., 0., 0., 0., 0., 0.],
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/pyoperators-0.13.6/.pybuild/pythonX.Y_2.7/build/test/test_fft.py", line 18, in func
    convol = ConvolutionOperator(kernel, image.shape)
  File "pyoperators/fft.py", line 100, in __init__
    nthreads = nthreads or FFTW_DEFAULT_NUM_THREADS
NameError: global name 'FFTW_DEFAULT_NUM_THREADS' is not defined

...

----------------------------------------------------------------------
Ran 2058 tests in 3.905s

FAILED (SKIP=9, errors=111)
E: pybuild pybuild:274: test: plugin distutils failed with: exit code=1: cd /pyoperators-0.13.6/.pybuild/pythonX.Y_2.7/build; python2.7 -m nose test
dh_auto_test: pybuild --test --test-nose -i python{version} -p 2.7 --dir . returned exit code 13

Full build log:
https://reproducible.debian.net/rb-pkg/unstable/amd64/pyoperators.html

-- System Information:
Debian Release: stretch/sid
APT prefers unstable
APT policy: (500, 'unstable')
Architecture: amd64 (x86_64)