configure.ac references the m4 directory so things like gtkdocize
will try and place m4 files there. We need to ensure the directory
exists or the builds can fail.
(From OE-Core rev: 0ced649c055dd25c500d0fd34e4ea94504d98580)
(From OE-Core rev: ce4cc1e8b48fb368fa96f63ee43a35d6ed142b21)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>