Unit-tests fail to build due to the following errors:

/home/cfx/Dev/google-benchmark/benchmark.git/test/string_util_gtest.cc:12:5: required from here
/home/cfx/Applications/googletest-1.8.1/include/gtest/gtest.h:1444:11: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
   if (lhs == rhs) {
       ~~~~^~~~~~

Fixes #741