Complete quoting for parameters in "configure.ac"

I recommend to be more explicit where macro expansion will not be required by [your build specification](https://github.com/bareos/bareos/blob/8db89da5bc744539874923b5e1415b248ce7228e/autoconf/configure.in#L7).
- [Quoting Text for Autoconf](http://autogen.sourceforge.net/acquoting.html)
- [On Quoting Arguments to macros](http://www.gnu.org/software/autoconf/manual/m4/Quoting-Arguments.html)
