To fix error: command 'clang' failed with exit status 1 when running `pip install pycurl', install Command Line Tools from Xcode.

1. Open Xcode
2. Go to Preferences (Command + ,)
3. Select the Downloads tab / Components subtab
4. Install Command Line Tools

Then pip install with ARCHFLAGS set:

$ env ARCHFLAGS="-arch x86_64" pip install pycurl