Fix build in a directory other than source directory.

This allows to build Bumblebee like this:

  mkdir /tmp/bb
  cd /tmp/bb
  /path/to/source/Bumblebee/configure
  make

The advantage is a clear separation of the source and build trees (which
allows for several build dirs with different compile flags for instance).
Merge pull request #316 from kroki/separate-build-dir

Fix builds outside of the source tree