fix(warnings): do not ignore return from system

eo/test/t-eoSelect.cpp

@@ -126,7 +126,7 @@ eoValueParam<unsigned> tournamentSizeParam = parser.createParam(unsigned(2), "to
       }
 
     // hard-coded directory name ...
-    system("mkdir ResSelect");
+    const int ignored = system("mkdir ResSelect");
     std::cout << "Testing the Selections\nParents size = " << pSize
 	 << ", offspring rate = " << oRate;
     std::cout << " and putting rsulting files in dir ResSelect" << std::endl;

eo/test/t-eoSharing.cpp

@@ -193,7 +193,7 @@ int the_main(int argc, char **argv)
 
       std::cout << "The resulting file (in dir ResSelect), contains \n";
     std::cout << " the empirical proba. for each indi to be selected." << std::endl;
-    system("mkdir ResSelect");
+    const int ignored = system("mkdir ResSelect");
 
     // initialize parent population
     parentsOrg.resize(pSize);