Got this solution from Stack Overflow.
Basically, previously had the following error:
_imagingft.c:73:10: fatal error: 'freetype/fterrors.h' file not found #include <freetype/fterrors.h> ^ 1 error generated.
You need freetype installed (use brew). But that creates the freetype2 library. So just do a
ln -s /usr/local/include/freetype2/ /usr/local/include/freetype
And then you'll be able to do the following successfully!
pip install -U PIL --allow-external PIL --allow-unverified PIL