title: Cannot install rpy2 in python 2.7
body: <p>I am attempting to install <code>RPy2</code> in the Python environment (<code>2.7.x</code>) that comes bundled in Mac OS X Sierra (<code>10.12.6</code>), and as such, I am attempting to install the latest non-2.9.x version from Terminal using <code>pip</code> via the command:</p>
  
  <pre><code>pip install 'rpy2&lt;2.9.0' --user
  </code></pre>
  
  <p>The <code>--user</code> flag is there because I have installed Homebrew on a user which is normally not admin and for which I turned on admin rights briefly for the sole purpose of installing Homebrew.</p>
  
  <p>Every time I run this command, I get the following error:</p>
  
  <pre><code>Command "/usr/bin/python -u -c "import setuptools, tokenize;__file__='/private/var/folders/1x/d_trx3556sl61by_tp4093j80000gq/T/pip-build-uV52fY/rpy2/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /var/folders/1x/d_trx3556sl61by_tp4093j80000gq/T/pip-LoW5FN-record/install-record.txt --single-version-externally-managed --compile --user --prefix=" failed with error code 1 in /private/var/folders/1x/d_trx3556sl61by_tp4093j80000gq/T/pip-build-uV52fY/rpy2/
  </code></pre>
  
  <p>Looking back into the output, it shows the following at time of failure:</p>
  
  <pre><code>creating build/temp.macosx-10.12-intel-2.7/rpy/rinterface
  /usr/local/Cellar/gcc/7.2.0/bin/gcc-7 -fno-strict-aliasing -fno-common -dynamic -arch x86_64 -arch i386 -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 x86_64 -arch i386 -pipe -I./rpy/rinterface -I/Library/Frameworks/R.framework/Resources/include -c ./rpy/rinterface/r_utils.c -o build/temp.macosx-10.12-intel-2.7/./rpy/rinterface/r_utils.o
  unable to execute '/usr/local/Cellar/gcc/7.2.0/bin/gcc-7': No such file or directory
  error: command '/usr/local/Cellar/gcc/7.2.0/bin/gcc-7' failed with exit status 1
  </code></pre>
  
  <p>I am unable to find anyone who is getting a similar error, and I cannot figure out what is going on.
  Does anyone know why this is failing?</p>