It does build them on install time, if a suitable compiler is available. A classic example is the `ws` module (WebSockets), when you install that you'll normally see the build log for its native dependencies on the npm output:

  $ npm install ws
  
  > [email protected] install /Users/coreh/Projects/test/node_modules/bufferutil
  > node-gyp rebuild
  
    CXX(target) Release/obj.target/bufferutil/src/bufferutil.o
    SOLINK_MODULE(target) Release/bufferutil.node
  
  > [email protected] install /Users/coreh/Projects/test/node_modules/utf-8-validate
  > node-gyp rebuild
  
    CXX(target) Release/obj.target/validation/src/validation.o
    SOLINK_MODULE(target) Release/validation.node
  /Users/coreh/Projects/test
  └─┬ [email protected] 
    ├─┬ [email protected] 
    │ ├── [email protected] 
    │ └── [email protected] 
    ├── [email protected] 
    ├── [email protected] 
    └── [email protected]