Conditionally switch between source maps and minification based on a gulp flag.

gulpfile.js

// `gulp stylus --prod` will enable "production mode"
// `gulp stylus` will leave the prod flag undefined, so it will do a dev build
var gulp  = require('gulp'),
    gutil = require('gulp-util'),
    prod  = gulp.env.prod;

gulp.task('stylus', ['cleancss'], function () {
  var stylus = require('gulp-stylus'),
      prefix = require('gulp-autoprefixer'),
      minify = require('gulp-minify-css');

  return gulp.src('./styl/main.styl')
      .pipe(stylus({linenos: !prod}))
      .pipe(prod ? prefix() : gutil.noop())
      .pipe(prod ? minify() : gutil.noop())
      .pipe(gulp.dest('./dist/css'));
});