Warn on npm install if Bower isn't installed.

build/bower-install.js

+var installer,
+	which = require( "which" ),
+	spawn = require( "child_process" ).spawn;
+
+try {
+	which.sync( "bower" );
+} catch( error ) {
+	console.error( "Bower must be installed to build jQuery." );
+	console.error( "Please install Bower by running the following command:" );
+	console.error( "npm install -g bower" );
+	process.exit( 1 );
+}
+
+installer = spawn( "bower", [ "install" ] );
+installer.stdout.on( "data", function( data ) {
+	console.log( data );
+});
+installer.stderr.on( "data", function( data ) {
+	console.error( data );
+});
+installer.on( "close", function( code ) {
+	process.exit( code );
+});

package.json

@@ -38,7 +38,8 @@
 		"grunt-jsonlint": "~1.0.0",
 		"gzip-js": "0.3.2",
 		"testswarm": "~1.1.0",
-		"requirejs": "~2.1.8"
+		"requirejs": "~2.1.8",
+		"which": "~1.0.5"
 	},
 	"scripts": {
 		"install": "bower install",