Bugfix for compiling C++ benchmark with g++.

Default command gave me a lot of unresolved references errors:

/tmp/ccWwYuqm.o: In function `GetLibrarayFunction(std::string)':
main.cpp:(.text+0x1f): undefined reference to `dlopen'
main.cpp:(.text+0x6b): undefined reference to `dlsym'
main.cpp:(.text+0x74): undefined reference to `dlerror'
collect2: error: ld returned 1 exit status