[Xcb] [PATCH util-common] config: ignore m4 files from libtool.

[Arnaud Fontaine]

Hi,

    >  The list  is taken  from the  defaults for  xorg which  have been
    > stable for the last two years.

    > This patch depends on a xcb/util patch as the subproject commit is
    > changed.

Pushed, thanks.

Cheers,
Arnaud