hide default-on benchmarks behind new default-off flag

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -231,7 +231,7 @@ if (BUILD_TESTING)
   add_subdirectory(test)
 endif()
 
-if (NOT CMAKE_CROSSCOMPILING)
+if (BUILD_BENCHMARKING)
   add_subdirectory(benchmark)
 endif()
 
