suppress 'unused variable' warnings for benchmark dummy variables

6 jobs for cpp11
Status Job ID Name Coverage
  Test
passed #240
Linux
clang-c++11

00:11:54

passed #242
Linux
clang-c++98

00:08:47

passed #239
Linux
gcc-c++11

00:16:01

passed #241
Linux
gcc-c++98

00:13:28

passed #243
Apple
macos-c++11

00:05:41

passed #244
Apple
macos-c++98

00:05:04