Compiling App for older OS X versions

Compiling App for older OS X versions

I recently submitted (my first) app to the AppStore. It works on OS X 10.8
and uses some of the 10.8 features like ShareKit. I wanted to support OS X
10.7 also, to make it available for 10.7 users. Of course, this would mean
that the 10.7 version will not have the ShareKit features. But I'm not
sure what compile settings to use to make it available for both, so that
10.8 users will be able to make use of the features and 10.7 users will
not see them.
I tried changing the Base SDK to 10.7, but it won't compile on that.
I tried changing the deployment target to 10.7 keeping the Base SDK 10.8.
It compiled, but I'm not sure if that is the right thing to do. Don't have
a 10.7 machine to test it, either.
If case 2 is correct, how do I check the in the code and make the menu
items disabled?