The benchmark library is compiled as C++11, but certain
tests are compiled as C++03. When -flto is enabled GCC 5.4
and above will diagnose an ODR violation in libstdc++'s <map>.

This ODR violation, although real, should likely be benign. For
this reason it seems sensible to simply suppress -Wodr when building
the C++03 test.

This patch fixes #420 and supersede's PR #424.