When attempting to use the latest appcompat-v7 support library in my project, I get the following error:
/Users/greg/dev/mobile/android_project/app/build/int
In case you don't want to use API 21 as the target API, and thus you don' t want to use the Material Theme, you have to use an older revision belonging to API 19 >
compile "com.android.support:appcompat-v7:19.0.+"
This also helps solving your problem, it only depends on what you want to achieve.
After creating a second project in the workspace in eclipse, I had this problem. I believe it is because I created it with a different SDK version and this ovewrote the android-support-v7-appcompat library.
I tried to clean everything up but to no avail. Ultimately, the suggestion above to edit project.properties and change target=android-21 and set my project to Android 5.0, fixed it.