Make it possible to create a virtualenv without --no-site-packages

[njsmith]

(This is sort of a companion to #217, see that for context.)

So I understand that installing of extensions which have dependencies on external C libraries is not really supported (except for the limited set of C libraries listed in the FAQ, I guess). However, some of these extensions are already installed system-wide, like numpy, which is super-handy because it avoids that whole C extension mess without causing any big head-aches for you guys.

I'd like to be able to use those pre-installed C extensions where 'pip install' doesn't work, and I'd also like to 'pip install' some other libraries that don't have external C dependencies.

But currently, it looks like you create virtualenvs with --no-site-packages, which means that I'm forced to choose: I can either have pre-installed C extensions, or extra pip installed packages, but not both. It'd be great if you either could stop using --no-site-packages, or have a checkbox to control it.

[njsmith]

It looks like virtualenv has changed its defaults in newer releases, so depending on what version you're using you might have to say "--system-site-packages" explicitly to request access to system-installed packages.