How to combine postcss / autoprefixer with LESS - less

Question

I have a site for which I use custom LESS stylesheets and they include Bootstrap LESS stylesheets. It is all compiled to the final style.css file with a style.css.map sourcemap.
This is how my Gulpfile looks:
var gulp = require('gulp');
var less = require('gulp-less-sourcemap');
var mySite = '.';
gulp.task('less', function () {
gulp.src(mySite + '/css/**.less')
.pipe(less())
.pipe(gulp.dest(mySite + '/css'));
});
Now I'd like to add autoprefixer. This is what I've tried but it doesn't work:
var gulp = require('gulp');
var less = require('gulp-less-sourcemap');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer-core');
var mySite = '.';
gulp.task('less', function () {
gulp.src(mySite + '/css/**.less')
.pipe(less())
.pipe(postcss([autoprefixer({ browsers: ['last 2 version'] })]))
.pipe(gulp.dest(mySite + '/css'));
});
It's because I am piping the generated sourcemaps into postcss and it cannot handle it. So I've tried this:
var gulp = require('gulp');
var less = require('gulp-less-sourcemap');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer-core');
var mySite = '.';
gulp.task('less', function () {
gulp.src(mySite + '/css/**.less')
.pipe(less())
.pipe(postcss([autoprefixer({ browsers: ['last 2 version'] })]))
.pipe(gulp.dest(mySite + '/css'));
gulp.src(mySite + '/css/**.css')
.pipe(postcss([autoprefixer({ browsers: ['last 2 version'] })]))
.pipe(gulp.dest(mySite + '/css'));
});
However, the prefixes are still not added. Maybe it's because the Bootstrap's CSS is already prefixed and it's somehow a problem for postcss?
How to make this work?

Answer 1

According to the Gulp-Less Readme,
If you are using version 1.3.7 or higher you will have the ability to
use a growing set LESS plugins, potentially simplifying your build
steps.
And the code they suggest using would look like this.
var LessPluginAutoPrefix = require('less-plugin-autoprefix'),
autoprefix= new LessPluginAutoPrefix({browsers: ["last 2 versions"]});
gulp.src('./less/**/*.less')
.pipe(less({
plugins: [autoprefix]
}))
.pipe(gulp.dest('./public/css'));