Create Build directory if the directory does not exist

[hinkley-dev]

I'd like to add

   if (!fs.existsSync(__dirname + '/build')){
    fs.mkdirSync(__dirname + '/build');
}

above line 22 of postInstall.js. When creating the file ..(__dirname, './build/jvm_dll_path.json) I get an error because the empty directory does not exists.

I'd like to create a PR to add this at it should not affect any other changes and will fix the case where the build directory does not exist when using npm to install.