cmake BSON_HAVE_STRNLEN examples

mongodb/mongo-c-driver src/libbson/CMakeLists.txt :100 

check_symbol_exists (strnlen string.h BSON_HAVE_STRNLEN)

realm/realm-core src/external/bson/CMakeLists.txt :45 

check_symbol_exists (strnlen string.h BSON_HAVE_STRNLEN)

cvet/fonline ThirdParty/mongo-c-driver/src/libbson/CMakeLists.txt :104 

check_symbol_exists (strnlen string.h BSON_HAVE_STRNLEN)