Use std::isinf() instead of isinf()

Audacity fails to build on Ubuntu 16.04 with g++ 5.3.1 (using C++11),
because isinf() is not defined. Thus use std::isinf() instead.

src/effects/Contrast.cpp

@@ -25,7 +25,7 @@
 #include "../widgets/NumericTextCtrl.h"
 #include "FileDialog.h"
 
-#include <math.h>
+#include <cmath>
 #include <limits>
 
 #if defined(__WXMSW__) && !defined(__CYGWIN__)
@@ -58,6 +58,8 @@
 #define DB_MAX_LIMIT 0.0   // TODO: We should probably fail WCAG2 if audio is massively distorted.
 #define WCAG2_PASS 20.0    // dB difference required to pass WCAG2 test.
 
+using std::isinf;
+
 bool ContrastDialog::GetDB(float &dB)
 {
    float rms = float(0.0);

src/effects/nyquist/Nyquist.cpp

@@ -27,8 +27,8 @@ effects from this one class.
 #include "../../Audacity.h"
 
 #include <algorithm>
+#include <cmath>
 
-#include <math.h>
 #include <locale.h>
 
 #include <wx/checkbox.h>
@@ -683,7 +683,7 @@ bool NyquistEffect::Process()
                // with very small values, bandwidth calculation may be inf.
                // (Observed on Linux)
                double bw = log(mF1 / mF0) / log(2.0);
-               if (!isinf(bw)) {
+               if (!std::isinf(bw)) {
                   bandwidth.Printf(wxT("(float %s)"), Internat::ToString(bw).c_str());
                }
             }