[Qgis-developer] Master_2: error: ‘isinf’ was not declared in this scope

[René-Luc Dhont]

Hi devs,

I have this error during the qgis master_2 building:
```
../src/gui/qgsscalerangewidget.cpp:93:21: error: ‘isinf’ was not 
declared in this scope
    if ( isinf( scale ) )
                      ^
../src/gui/qgsscalerangewidget.cpp:93:21: note: suggested alternative:
In file included from /usr/include/c++/5/random:38:0,
                  from /usr/include/c++/5/bits/stl_algo.h:66,
                  from /usr/include/c++/5/algorithm:62,
                  from /usr/include/qt4/QtCore/qglobal.h:68,
                  from /usr/include/qt4/QtCore/qnamespace.h:45,
                  from /usr/include/qt4/QtCore/qobjectdefs.h:45,
                  from /usr/include/qt4/QtCore/qobject.h:47,
                  from /usr/include/qt4/QtGui/qlayout.h:45,
                  from /usr/include/qt4/QtGui/qgridlayout.h:45,
                  from /usr/include/qt4/QtGui/QGridLayout:1,
                  from ../src/gui/qgsscalerangewidget.h:19,
                  from ../src/gui/qgsscalerangewidget.cpp:16:
/usr/include/c++/5/cmath:621:5: note:   ‘std::isinf’
      isinf(_Tp __x)
      ^
../src/gui/qgsscalerangewidget.cpp: In member function ‘void 
QgsScaleRangeWidget::setMaximumScale(double)’:
../src/gui/qgsscalerangewidget.cpp:105:21: error: ‘isinf’ was not 
declared in this scope
    if ( isinf( scale ) )
                      ^
../src/gui/qgsscalerangewidget.cpp:105:21: note: suggested alternative:
In file included from /usr/include/c++/5/random:38:0,
                  from /usr/include/c++/5/bits/stl_algo.h:66,
                  from /usr/include/c++/5/algorithm:62,
                  from /usr/include/qt4/QtCore/qglobal.h:68,
                  from /usr/include/qt4/QtCore/qnamespace.h:45,
                  from /usr/include/qt4/QtCore/qobjectdefs.h:45,
                  from /usr/include/qt4/QtCore/qobject.h:47,
                  from /usr/include/qt4/QtGui/qlayout.h:45,
                  from /usr/include/qt4/QtGui/qgridlayout.h:45,
                  from /usr/include/qt4/QtGui/QGridLayout:1,
                  from ../src/gui/qgsscalerangewidget.h:19,
                  from ../src/gui/qgsscalerangewidget.cpp:16:
/usr/include/c++/5/cmath:621:5: note:   ‘std::isinf’
      isinf(_Tp __x)
      ^
```