pillow: install fails without CLT

Otherwise I get this error

    ==> Installing pillow
    ==> Downloading https://github.com/python-imaging/Pillow/archive/2.4.0.tar.gz
    ######################################################################## 100.0%
    ==> python setup.py install --prefix=/usr/local/Cellar/pillow/2.4.0 --record=installed.txt --single-version-external
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include/tk.h:78:11: fatal error: 'X11/Xlib.h' file not found
    #       include <X11/Xlib.h>
                    ^
    1 error generated.
    error: command 'clang' failed with exit status 1
    
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