Actually, everything ends up in include/benchmark/

    %%%%> make install
    [ 85%] Built target benchmark
    [100%] Built target benchmark_test
    Install the project...
     -- Install configuration: ""
     -- Installing: /usr/local/lib/libbenchmark.a
     -- Installing: /usr/local/include/benchmark
     -- Up-to-date: /usr/local/include/benchmark/benchmark.h
     -- Up-to-date: /usr/local/include/benchmark/macros.h