[Dune] m4 cleanup

[Christian Engwer]

Hi Sven,

> > $() is a bashism and `` is posix-sh.
> 
> you are wrong with this one:
> 
>     http://opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06_03
> 
> $() _is_ POSIX and is generally considered to be the better solution.
> It is much more flexible -- for example it can be nested, which is
> impossible with ``.  There are other advantages as well, but I forgot
> what exactly they are.

You are right and I am wrong, but still I remember that there was a
reason not to use it for portable code and now I found the reference again:

http://www.linuxtopia.org/online_books/linux_development_tools/autoconf_guide/autoconf_reference_guide_130.html

The autoconf guide states that certain shell implementations, e.g. on
solaris, do not support $() although it is posix.

Christian