Try to reuse installed software

[rauljim]

I suppose some people have their own Android setup and they would rather keep it.

It'd be great if setup-linux-environment.sh asked if the user wants to re-use an already installed package. For instance, the android-sdk.

This is how I imagine it:
$ ./setup-linux-environment.sh
I couldn't find Android SDK in your system. You can:

1. Press ENTER to automatically download and install it
2. Type the path to your android-sdk-linux .

[whirm]

Yes, that would be cool.
It's on my TODO list :)

[whirm]

If anyone is willing to take over that ticket, please update both setup-linux-environment.sh and build-python-m2crypto.sh