如何在 Mac OS 上使用 Pip 安装 PIL？

我尝试使用以下命令安装PIL（Python Imaging Library）：

sudo pip install pil

但我收到以下信息：

Downloading/unpacking PIL
  You are installing a potentially insecure and unverifiable file. Future versions of pip will default to disallowing insecure files.
  Downloading PIL-1.1.7.tar.gz (506kB): 506kB downloaded
  Running setup.py egg_info for package PIL
    WARNING: '' not a valid package name; please use only.-separated package names in setup.py
Installing collected packages: PIL
  Running setup.py install for PIL
    WARNING: '' not a valid package name; please use only.-separated package names in setup.py
    --- using frameworks at /System/Library/Frameworks
    building '_imaging' extension
    clang -fno-strict-aliasing -fno-common -dynamic -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -mno-fused-madd -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -arch i386 -arch x86_64 -pipe -IlibImaging -I/System/Library/Frameworks/Python.framework/Versions/2.7/include -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c _imaging.c -o build/temp.macosx-10.8-intel-2.7/_imaging.o
    unable to execute clang: No such file or directory
    error: command 'clang' failed with exit status 1
    Complete output from command /usr/bin/python -c "import setuptools;__file__='/private/tmp/pip_build_root/PIL/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-AYrxVD-record/install-record.txt --single-version-externally-managed:
    WARNING: '' not a valid package name; please use only.-separated package names in setup.py
running install
running build
.
.
.
.
copying PIL/XVThumbImagePlugin.py -> build/lib.macosx-10.8-intel-2.7
running build_ext
--- using frameworks at /System/Library/Frameworks
building '_imaging' extension
creating build/temp.macosx-10.8-intel-2.7
creating build/temp.macosx-10.8-intel-2.7/libImaging
clang -fno-strict-aliasing -fno-common -dynamic -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -mno-fused-madd -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -arch i386 -arch x86_64 -pipe -IlibImaging -I/System/Library/Frameworks/Python.framework/Versions/2.7/include -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c _imaging.c -o build/temp.macosx-10.8-intel-2.7/_imaging.o
unable to execute clang: No such file or directory
error: command 'clang' failed with exit status 1
----------------------------------------
Cleaning up…

请帮我安装PIL，谢谢。