Skip warning compiler flags checks in CMake when NVBench is consumed as a dependency

[bernhardmgruber]

When NVBench is consumed as a dependency (for example, from CCCL), cmake still checks for a bunch of warning flags:

-- Performing Test NVBench_CXX_FLAG__Wall
-- Performing Test NVBench_CXX_FLAG__Wall - Success
-- Performing Test NVBench_CXX_FLAG__Wextra
-- Performing Test NVBench_CXX_FLAG__Wextra - Success
-- Performing Test NVBench_CXX_FLAG__Wconversion
-- Performing Test NVBench_CXX_FLAG__Wconversion - Success
-- Performing Test NVBench_CXX_FLAG__Woverloaded_virtual
-- Performing Test NVBench_CXX_FLAG__Woverloaded_virtual - Success
-- Performing Test NVBench_CXX_FLAG__Wcast_qual
-- Performing Test NVBench_CXX_FLAG__Wcast_qual - Success
-- Performing Test NVBench_CXX_FLAG__Wpointer_arith
-- Performing Test NVBench_CXX_FLAG__Wpointer_arith - Success
-- Performing Test NVBench_CXX_FLAG__Wunused_local_typedef
-- Performing Test NVBench_CXX_FLAG__Wunused_local_typedef - Failed
-- Performing Test NVBench_CXX_FLAG__Wunused_parameter
-- Performing Test NVBench_CXX_FLAG__Wunused_parameter - Success
-- Performing Test NVBench_CXX_FLAG__Wvla
-- Performing Test NVBench_CXX_FLAG__Wvla - Success
-- Performing Test NVBench_CXX_FLAG__Wgnu
-- Performing Test NVBench_CXX_FLAG__Wgnu - Failed
-- Performing Test NVBench_CXX_FLAG__Wno_gnu_line_marker
-- Performing Test NVBench_CXX_FLAG__Wno_gnu_line_marker - Success

This seems useful when NVBench is built for testing, but seems unncessary when NVBench is consumed as dependency. I suggest to skip these checks in the latter case.