cairocffi: "No package 'libffi' found" error during pip install

When I tried to pip install cairocffi into a fresh virtualenv, I got an error saying “Package libffi was not found…”

$ mkvirtualenv tmp
New python executable in tmp/bin/python
Installing setuptools............done.
Installing pip...............done.

(tmp)$ pip install cairocffi
Downloading/unpacking cairocffi
  Downloading cairocffi-0.5.tar.gz (70kB): 70kB downloaded
  Running setup.py egg_info for package cairocffi

    no previously-included directories found matching 'docs/_build'
Downloading/unpacking cffi>=0.6 (from cairocffi)
  Downloading cffi-0.6.tar.gz (169kB): 169kB downloaded
  Running setup.py egg_info for package cffi
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found

Downloading/unpacking pycparser (from cffi>=0.6->cairocffi)
  Downloading pycparser-2.09.1.tar.gz (201kB): 201kB downloaded
  Running setup.py egg_info for package pycparser

Installing collected packages: cairocffi, cffi, pycparser
  Running setup.py install for cairocffi

    no previously-included directories found matching 'docs/_build'
  Running setup.py install for cffi
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    building '_cffi_backend' extension
    /usr/bin/gcc-4.2 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -pipe -no-cpp-precomp -pipe -arch x86_64 -I/usr/include/ffi -I/usr/include/libffi -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c c/_cffi_backend.c -o build/temp.macosx-10.7-x86_64-2.7/c/_cffi_backend.o
    llvm-gcc-4.2 -Wl,-F. -bundle -undefined dynamic_lookup -Wl,-F. -bind_at_load -pipe -no-cpp-precomp -arch x86_64 build/temp.macosx-10.7-x86_64-2.7/c/_cffi_backend.o -lffi -o build/lib.macosx-10.7-x86_64-2.7/_cffi_backend.so

  Running setup.py install for pycparser

Successfully installed cairocffi cffi pycparser
Cleaning up...

(tmp) $ python --version
Python 2.7.1