Make it easier to install when the re2 library is in a virtualenv.

[akumria]

Python virtual environments (virtualenv) can have private library and include directories. If we detect that we are inside a virtualenv (with the VIRTUAL_ENV environment variable) add it to the list of paths to search for the re2 library.

Installing the re2 library in a virtualenv is basically:

make
make test
make install prefix=$VIRTUAL_ENV