bootstrap.conf: add check for flex
bootstrap.conf: add check for flex

We need the flex to generate new xen_xl_disk parser.

Signed-off-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

VenkatDatta/libvirt,agx/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,datto/libvirt,fabianfreyer/libvirt,datto/libvirt,VenkatDatta/libvirt,taget/libvirt,shugaoye/libvirt,rlaager/libvirt,elmarco/libvirt,jardasgit/libvirt,eskultety/libvirt,rlaager/libvirt,datto/libvirt,olafhering/libvirt,eskultety/libvirt,jardasgit/libvirt,rlaager/libvirt,shugaoye/libvirt,elmarco/libvirt,datto/libvirt,rlaager/libvirt,olafhering/libvirt,taget/libvirt,eskultety/libvirt,taget/libvirt,agx/libvirt,nertpinx/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,libvirt/libvirt,agx/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt,agx/libvirt,crobinso/libvirt,VenkatDatta/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,jardasgit/libvirt,zippy2/libvirt,agx/libvirt,shugaoye/libvirt,jfehlig/libvirt,jardasgit/libvirt,taget/libvirt,eskultety/libvirt,crobinso/libvirt,rlaager/libvirt,eskultety/libvirt,elmarco/libvirt,datto/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,elmarco/libvirt,elmarco/libvirt,jardasgit/libvirt,shugaoye/libvirt,olafhering/libvirt,taget/libvirt,olafhering/libvirt,zippy2/libvirt,andreabolognani/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jfehlig/libvirt,crobinso/libvirt