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Fix unit tests compilation with non-gnu / non-msvc compilers with c++11 support. (#1691)

donotoptimize_test.cc could not be compiled under non-gnu / non-msvc compilers,
because only deprecated version of DoNotOptimize is available for these
compilers. Tests are compiled with -Werror. Patch fixes test compilation by
providing non-deprecated version of DoNotOptimize for compilers with c++11
standard support.

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>