The symptom for the end-user looks kind of weird:

    running build_ext
building 'Cython.Plex.Scanners' extension
/ A p p l i c a t i o n s / X c o d e . a p p / C o n t e n t s / D e v e l o p e r / T o o l c h a i n s / X c o d e D e f a u l t . x c t o o l c h a i n / u s r / b i n / c l a n g   - f n o - s t r i c t - a l i a s i n g   - f n o - c o m m o n   - d y n a m i c   - I / h o m e b r e w / i n c l u d e   - I / h o m e b r e w / o p t / s q l i t e / i n c l u d e   - i s y s r o o t   / A p p l i c a t i o n s / X c o d e . a p p / C o n t e n t s / D e v e l o p e r / P l a t f o r m s / M a c O S X . p l a t f o r m / D e v e l o p e r / S D K s / M a c O S X 1 0 . 8 . s d k   - I / A p p l i c a t i o n s / X c o d e . a p p / C o n t e n t s / D e v e l o p e r / P l a t f o r m s / M a c O S X . p l a t f o r m / D e v e l o p e r / S D K s / M a c O S X 1 0 . 8 . s d k / S y s t e m / L i b r a r y / F r a m e w o r k s / T k . f r a m e w o r k / V e r s i o n s / 8 . 5 / H e a d e r s   - D N D E B U G   - g   - f w r a p v   - O 3   - W a l l   - W s t r i c t - p r o t o t y p e s   -I/homebrew/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c Cython/Plex/Scanners.c -o build/temp.macosx-10.8-x86_64-2.7/Cython/Plex/Scanners.o
unable to execute /: Permission denied
error: command '/' failed with exit status 1


Pasted here to help googlers to find it :-)