Smoke-test externs syntax at build time

[martinklepsch]

This could be done by compiling a minimal project with the supplied externs (probably cljs isn't even needed). The result would be that syntax errors in externs and duplicate declarations are catched at build time compared to run time.

via cljsjs/packages#508

[Deraen]

This is implemented by https://github.com/cljsjs/packages/blob/master/test-extern-files.sh which is running on CircleCI.