cleaning: remove useless variables in build sub
Allow using functions refs in default config

It is now possible to put a function reference in the default
configuration. When accessing a configuration option (or one of the
elements in the path) that is a function, the function is executed
and the return of the function is used as the value. The function
takes as first argument the project's name, and on other arguments
the path to the config option.
Fix lookup of '0' or empty options
Make the describe option a perl function

The describe option is now a perl function. This allows us to remove
some explicit calls to the git_describe function, and make sure that
it is called any time the describe values are used. This makes the
"mkpkg showconf [project] rpmspec" command work in all cases (before
that there could be problems as the describe option was not set).