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Homebrew build fails with "fatal error: 'ffi.h' file not found"

See original GitHub issue

When I try to install conan from homebrew I get the following error

$ brew install conan
==> Downloading https://pypi.python.org/packages/fa/c6/f94d46dd2b958248114a86a8952882e67a6ace55c72b8600fd30bf85ce7f/co
Already downloaded: /Users/sam/Library/Caches/Homebrew/conan-0.10.0.tar.gz
==> Downloading https://pypi.python.org/packages/source/s/setuptools/setuptools-20.9.0.tar.gz
Already downloaded: /Users/sam/Library/Caches/Homebrew/conan--setuptools-20.9.0.tar.gz
==> python -c import setuptools... --no-user-cfg install --prefix=/Users/sam/.brew/Cellar/conan/0.10.0/libexec/vendor 
==> Downloading https://pypi.python.org/packages/d3/30/54c6dab05a4dec44db25ff309f1fbb6b7a8bde3f2bade38bb9da67bbab8f/ba
Already downloaded: /Users/sam/Library/Caches/Homebrew/conan--backport_ipaddress-0.1.tar.gz
==> python -c import setuptools... --no-user-cfg install --prefix=/Users/sam/.brew/Cellar/conan/0.10.0/libexec/vendor 
==> Downloading https://pypi.python.org/packages/e5/6e/13022066f104f6097a7414763c5658d68081ad0bc2b0630a83cd498a6f22/bo
Already downloaded: /Users/sam/Library/Caches/Homebrew/conan--boto-2.38.0.tar.gz
==> python -c import setuptools... --no-user-cfg install --prefix=/Users/sam/.brew/Cellar/conan/0.10.0/libexec/vendor 
==> Downloading https://pypi.python.org/packages/d2/59/e61e3dc47ed47d34f9813be6d65462acaaba9c6c50ec863db74101fa8757/bo
Already downloaded: /Users/sam/Library/Caches/Homebrew/conan--bottle-0.12.9.tar.gz
==> python -c import setuptools... --no-user-cfg install --prefix=/Users/sam/.brew/Cellar/conan/0.10.0/libexec/vendor 
==> Downloading https://pypi.python.org/packages/b6/98/11feff87072e2e640fb8320712b781eccdef05d588618915236b32289d5a/cf
Already downloaded: /Users/sam/Library/Caches/Homebrew/conan--cffi-1.6.0.tar.gz
==> python -c import setuptools... --no-user-cfg install --prefix=/Users/sam/.brew/Cellar/conan/0.10.0/libexec/vendor 
Last 15 lines from /Users/sam/Library/Logs/Homebrew/conan/05.python:
copying cffi/vengine_gen.py -> build/lib.macosx-10.12-intel-2.7/cffi
copying cffi/verifier.py -> build/lib.macosx-10.12-intel-2.7/cffi
copying cffi/_cffi_include.h -> build/lib.macosx-10.12-intel-2.7/cffi
copying cffi/parse_c_type.h -> build/lib.macosx-10.12-intel-2.7/cffi
copying cffi/_embedding.h -> build/lib.macosx-10.12-intel-2.7/cffi
running build_ext
building '_cffi_backend' extension
creating build/temp.macosx-10.12-intel-2.7
creating build/temp.macosx-10.12-intel-2.7/c
clang -fno-strict-aliasing -fno-common -dynamic -arch i386 -arch x86_64 -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -arch i386 -arch x86_64 -pipe -DUSE__THREAD -I/usr/include/ffi -I/usr/include/libffi -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c c/_cffi_backend.c -o build/temp.macosx-10.12-intel-2.7/c/_cffi_backend.o
c/_cffi_backend.c:15:10: fatal error: 'ffi.h' file not found
#include <ffi.h>
         ^
1 error generated.
error: command 'clang' failed with exit status 1

READ THIS: https://git.io/brew-troubleshooting
If reporting this issue please do so at (not Homebrew/brew):
  https://github.com/Homebrew/homebrew-core/issues