Fix build on gcc 5

[minaevmike]

fix #135
Also i have some problems with building tests:

In file included from /root/prog/mblackhole/build/foreign/google-benchmark/include/benchmark/benchmark.h:18:0,
                 from /root/prog/mblackhole/bench/attribute.cpp:1:
/root/prog/mblackhole/build/foreign/google-benchmark/include/benchmark/benchmark_api.h: In function ‘void blackhole::benchmark::from_owned(benchmark::State&)’:
/root/prog/mblackhole/build/foreign/google-benchmark/include/benchmark/benchmark_api.h:211:54: error: inconsistent operand constraints in an ‘asm’
     asm volatile("" : "+rm" (const_cast<Tp&>(value)));

But i can't fix it because google-benchmark download from your aws cloud=) i try to use google-benchmark from git master and it works fine