install.sh: prevent Vim from catching deadly signals

On OSX, linking YCM against the Homebrew version of Python, while using
the Homebrew version of Vim results in the following when attempting to
start Vim:

    Vim: Caught deadly signal ABRT
    Vim: Finished.

To fix this, perform OS detection (at the `uname -o' level) and use the
system Python when running cmake on YCM under OSX.