jQuery Mobile build with grunt

jQuery Mobile build with grunt

I have downloaded jQuery mobile with bower and now i am trying to build jquery with grunt as described on jquery-mobile github readme 
https://github.com/jquery/jquery-mobile
but i am getting error that ' build' directory is missing. 
after looking in the bower.json file
https://github.com/jquery/jquery-mobile/blob/master/bower.json
i saw that build directory is added into ignore list. 

now my question is, if that build directory is required to build the files then why its in ignore list? 
is there any way to to build files without that build directory?