Add policy CMP0056, which honors the link flags in try_compile and
try_run. This allows for building against non-system libc++ by providing
the correct -L and rpath options in a containing project.

For example:

    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -L ${LLVM_ROOT}/lib -l c++ -l c++abi")
    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,-rpath,${LLVM_ROOT}/lib")