libbsd / waf: Broken --show-commands support

[Christian Mauderer]

Hello,

I'm currently trying to port pf to libbsd. While doing that, I wanted to
check some of the compiler calls using the waf configure option
--show-commands. With the option set, every call went wrong.

I was able to track the problem down to the patches introduced with the
long command line support. It seems that the command is joined for
printing it and than the joined version is given to a python Popen call.
As far as I understand the python documentation, Popen expects an array.

The attached patch fixes this by using a temporary variable for the
string that gets printed. Could you review the patch to check that it
doesn't break the long command line support again?

Kind regards

Christian Mauderer