按照步骤如何安装 python 图像库 (PIL)？用于在 Ubuntu 14.04 上安装 PIL 的预设置库。但是，当我运行时pip install PIL --allow-external PIL --allow-unverified PIL看到以下错误：
building '_imagingft' extension
x86_64-linux-gnu-gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/freetype2 -IlibImaging -I/usr/include -I/usr/include/tcl8.6 -I/usr/local/include -I/usr/include/python2.7 -c _imagingft.c -o build/temp.linux-x86_64-2.7/_imagingft.o
_imagingft.c:73:31: fatal error: freetype/fterrors.h: No such file or directory
#include <freetype/fterrors.h>
^
compilation terminated.
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
----------------------------------------
Cleaning up...
Command /usr/bin/python -c "import setuptools, tokenize;__file__='/tmp/pip_build_ubuntu/PIL/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record …