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
  
  > bufferutil@1.2.1 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
  
  > utf-8-validate@1.2.1 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
  └─┬ ws@0.8.1 
    ├─┬ bufferutil@1.2.1 
    │ ├── bindings@1.2.1 
    │ └── nan@2.1.0 
    ├── options@0.0.6 
    ├── ultron@1.0.2 
    └── utf-8-validate@1.2.1