error occurs when installing cryptography for scrapy in virtualenv on OS X
<p>I was installing scrapy with <code>pip</code> in <code>virtualenv</code> on <code>OS X 10.11</code>. When it's installing cryptography, it said:</p>

<pre><code>building '_openssl' extension
cc -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 -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c build/temp.macosx-10.11-intel-2.7/_openssl.c -o build/temp.macosx-10.11-intel-2.7/build/temp.macosx-10.11-intel-2.7/_openssl.o
build/temp.macosx-10.11-intel-2.7/_openssl.c:423:10: fatal error: 'openssl/e_os2.h' file not found
#include &lt;openssl/e_os2.h&gt;
     ^
1 error generated.
error: command 'cc' failed with exit status 1
</code></pre>

<hr>

<pre><code>Command "/Users/Kris/Workspace/env/scrapy/bin/python -c "import setuptools, tokenize;__file__='/private/var/folders/wx/jm7lxhps3n7484zl4gv9s3180000gn/T/pip-build-2rJYgI/cryptography/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /var/folders/wx/jm7lxhps3n7484zl4gv9s3180000gn/T/pip-nXp2mE-record/install-record.txt --single-version-externally-managed --compile --install-headers /Users/Kris/Workspace/env/scrapy/include/site/python2.7/cryptography" failed with error code 1 in /private/var/folders/wx/jm7lxhps3n7484zl4gv9s3180000gn/T/pip-build-2rJYgI/cryptography
</code></pre>

<p>Any suggestions that could help me fix it?<br>
BTW, when I was installing <code>scrapy</code> without <code>virtualenv</code>, everything went well.</p>