take isinf and isnan from std, to avoid ambiguities

[[Imported from SVN: r11785]]

dune/solvers/solvers/loopsolver.cc

@@ -112,7 +112,7 @@ void LoopSolver<VectorType, BitVectorType>::solve()
         if (this->useRelativeError_ && !std::isnan(error/oldNorm))
             error = error / oldNorm;
 
-        if (!isinf(convRate) && !isnan(convRate) && i>0)
+        if (!std::isinf(convRate) && !std::isnan(convRate) && i>0)
         {
             totalConvRate *= convRate;
             this->maxTotalConvRate_ = std::max(this->maxTotalConvRate_, std::pow(totalConvRate, 1/((real_type)convRateCounter+1)));