Dev:C++11 Migration

Have a look to C++11Support branch for progress. C++11 Migration merged to master, + other fix.

C++11 compliance or Migration

• Should keep the possibility to build on C++98.

Enable C++ Support

• DONE : ADD to both "synfig-core/m4/" and "synfig-studio/m4/" the "ax_cxx_compile_stdcxx.m4" "and ax_cxx_compile_stdcxx_11.m4" directives from https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html
• DONE : synfig-core and synfig-studio / configure.ac : AX_CXX_COMPILE_STDCXX_11([noext],[optional])

Fix Code to be compliant

• DONE : replace isnan , isinf by std::isnan , std::isinf
• replace NULL/0 by nullptr (BOOST MACRO)
• DONE : update sigc++ --> only #include <sigc++/sigc++.h>
• FIXED : "lvalue required as unary '&' operand" errors (info [1] [2] [3] )
• DONE : fix warning for preprocessor strings

Useful information about C++11 Migration

• General information about C++11 : Stroustrup.b C++11 FAQ
• https://devcentral.f5.com/articles/case-study-linerates-c98-to-c11-migration