Simple, gulp config.

Gulpfile.js

var gulp = require('gulp');
var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var cssnano = require('cssnano');

gulp.task('js', function() {
  gulp.src('app/js/*.js')
    .pipe(concat('script.js'))
    .pipe(uglify())
    .pipe(gulp.dest('build/js/'))
});

gulp.task('css', function() {
  var preprocessors = [
    autoprefixer({browsers: ['last 2 version']}),
    cssnano(),
  ]
  gulp.src('app/css/*.css')
    .pipe(concat('main.css'))
    .pipe(postcss(preprocessors))
    .pipe(gulp.dest('build/css/'))
});

//gulp.watch('app/css/*.css', ['css']);

gulp.task('default', ['js', 'css'], function() {

});