Use CMake SET instead of string CONCAT
Use CMake SET instead of string CONCAT

string(CONCAT...) was not available in older versions of CMake
targeted by this project.

unittest-cpp/unittest-cpp,grahamreeds/unittest-cpp,pjohnmeyer/unittest-cpp,unittest-cpp/unittest-cpp,grahamreeds/unittest-cpp,grahamreeds/unittest-cpp,pjohnmeyer/unittest-cpp,unittest-cpp/unittest-cpp