[bugfix] Qualify with namespace std::

dune/solvers/solvers/cgsolver.cc

@@ -134,7 +134,7 @@ void CGSolver<MatrixType, VectorType>::solve()
 
         normOfOldCorrection = normOfCorrection;
 
-        if (!isinf(convRate) && !isnan(convRate)) {
+        if (!std::isinf(convRate) && !std::isnan(convRate)) {
             totalConvRate *= convRate;
             this->maxTotalConvRate_ = std::max(this->maxTotalConvRate_, std::pow(totalConvRate, 1/((double)convRateCounter+1)));
             convRateCounter++;