Refactor build directory setting into separate method and solve a few bugs.

* Move setting of new build directory from constructor to _set_builddir()
  method including detection if directory (current or source) it should be
  relative to.
* Even if a new build directory was specified, detect if it matches the source
  directory and unset it in such a case.
* Use _set_builddir() in enforce_out_of_source_tree() methods. Previous
  implementation didn't handle default build directory properly (i.e.
  relativeness to current or source directory).

Signed-off-by: Modestas Vainius <modestas@vainius.eu>