As of Eclipse 18.104.22.168, the build-on-save feature is not enabled by default. To fix this, make sure that your project is selected and navigate to:
Project > Properties > C/C++ Buildand you should see a screen like the following:
Click the Behavior tab on that screen:
Then check the Build on resource save checkbox:
Click the Apply and Ok buttons; then your project should immediately build, and rebuild each time you save.