In the dialogue window for External Tool Configurations under the 'Program' group is a tab Build.

In the Build tab is the check box 'Build before Launch'

I have created an External Tool with the 'Build before Launch' box checked and 'The Project containing the selected 
resource' button selected.

I have performed a 'Clean Project' before running the External Tool.

The External Tool runs fine, but the selected resource is not built before the tool runs.

Have I misunderstood the meaning of the 'Build before Launch'?