CMAKE_REQUIRED_FLAGS is intended to be added only to the compile for the
next probe, and inside an cmake_push_check_state/cmake_pop_check_state
guard. If we add it to the main flags (CMAKE_CXX_FLAGS), it will also be
added to subsequent checks, so it suffices.
We want to compile with the -std=c++0x since DUNE uses this settings and
we need to use the same ABI as that one if we are going to link with it.
Find module is a copy of the one in cmake/modules in (the cmake branch of)
dune-common.
