BLD: always put numpy in install_requires for wheels on OS X. Closes g…

…h-5184.

Note: when pip downloads a tarball it also runs bdist_wheel, so avoid adding
numpy to setup_requires/install_requires on other platforms.

setup.py

@@ -204,6 +204,11 @@ def setup_package():
     build_requires = []
     try:
         import numpy
+        if (len(sys.argv) >= 2 and sys.argv[1] == 'bdist_wheel' and
+                sys.platform == 'darwin'):
+            # We're ony building wheels for platforms where we know there's
+            # also a Numpy wheel, so do this unconditionally.  See gh-5184.
+            build_requires = ['numpy>=1.6.2']
     except:
         build_requires = ['numpy>=1.6.2']