synfig-core : c++11 support ( MUST BE VALIDATED OVER MACOSX )
    
    * Add M4 macros : AX_CXX_COMPILE_STDCXX and AX_CXX_COMPILE_STDCXX_11
    * Remplace isnan by std::isnan
    * Remplace isinf by std::isinf
    * MacOSX : I have removed (commented) the isnan MacOSX quick fix.
    When validated on MacOSX platform things above "Commented during c++11 support"
    could be removed.