Search in build/ sub-dir if only suite root is given

If the package suite was given (e.g. DUNE_ROOT=/blum), then the code set up the root for each individual package automatically (e.g. DUNE_COMMON_ROOT=/blum/dune-common), but the path which was then
activated did not get the local build sub-directory (e.g. if we are building opm-autodiff in /frub/opm-autodiff/build, then the local build directory is "build/"), and thus this was not appended to the library search path. The result was that the source was found (because the root pointed to a valid source tree), but the library was not (because it is "hidden" in the subdirectory).

This patch is created as a result of issue opm/opm-autodiff#59. Please hold the merge until @qilicun can confirm that it works properly in that setup.