Update cmake/NVBenchConfigTarget.cmake

Author: alliepiper

cmake/NVBenchConfigTarget.cmake

@@ -23,7 +23,7 @@ set_target_properties(nvbench.build_interface PROPERTIES
 
 function(nvbench_add_cxx_flag target_name type flag)
   string(MAKE_C_IDENTIFIER "NVBench_CXX_FLAG_${flag}" var)
-  if (NOT ${NVBench_TOPLEVEL_PROJECT})
+  if (NOT NVBench_TOPLEVEL_PROJECT)
     set(CMAKE_REQUIRED_QUIET ON)
   endif()
   check_cxx_compiler_flag(${flag} ${var})