Fix sourcemaps in gulp build

Pass correct path when writing and referencing sourcemap file to make it
work with external sourcemaps.

gulpfile.js

@@ -59,7 +59,11 @@ function processScripts(srcGlob, destDir, destFileName) {
 		.pipe(addSourcemaps ? sourcemaps.init() : noop())
 		.pipe(concat(destFileName))
 		.pipe(addSourcemaps ?
-			sourcemaps.write(args.embedsourcemaps ? undefined : '.', {includeContent: args.sourcemapsincludecontent, sourceRoot: prefix}) :
+			sourcemaps.write(args.embedsourcemaps ? undefined : '.', {
+				includeContent: args.sourcemapsincludecontent,
+				sourceRoot: prefix,
+				sourceMappingURLPrefix: path.relative(cfg.dirs.output, destDir)
+			}) :
 			noop())
 		.pipe(gulp.dest(destDir));
 }
@@ -75,7 +79,11 @@ function processStylesheets(srcGlob, destDir, destFileName) {
 			postcss([autoprefixer({overrideBrowserslist: ['last 2 versions']})]) :
 			noop())
 		.pipe(addSourcemaps ?
-			sourcemaps.write(args.embedsourcemaps ? undefined : '.', {includeContent: args.sourcemapsincludecontent, sourceRoot: prefix}) :
+			sourcemaps.write(args.embedsourcemaps ? undefined : '.', {
+				includeContent: args.sourcemapsincludecontent,
+				sourceRoot: prefix,
+				sourceMappingURLPrefix: path.relative(cfg.dirs.output, destDir)
+			}) :
 			noop())
 		.pipe(gulp.dest(destDir));
 }