Define macros only when necessary in cpp files

In workgen and the cpp testing framework, we define the following macros:

#define __STDC_LIMIT_MACROS  // needed to get UINT64_MAX in C++

#define __STDC_FORMAT_MACROS // needed to get PRIuXX macros in C++ 

They are required for older gcc versions. We should probably define them only if they are not already defined, i.e:

#ifndef __STDC_LIMIT_MACROS

#define __STDC_LIMIT_MACROS

#endif

#ifndef __STDC_FORMAT_MACROS

#define __STDC_FORMAT_MACROS

#endif 

Definition of done:

Check the cpp files compile using aws-perf from Jenkins.