Remove workaround for threading library bug in old gcc
Summary
This MR removes the old workaround that was added for the old gcc 4.8 due to a bug in the handling of the threading library.
Details
See https://stackoverflow.com/questions/19463602/compiling-multithread-code-with-g
The workaround is not needed for recent g++ versions any more.
The Threads cmake find-package is added to the DuneCommon macros and link_libraries is called to set the threading library support to all targets. This global link options is not optimal. It would be better to add this as link flags to a specific target and communicate the flat through linking against this target, e.g. dunecommon. This is not yet possible, since not all dune libraries and executables link against dunecommon but expect some flags and include dirs to be set globally. Whenever this is changed, this global link_libraries should be replaced with a local linking.