Grub Customizer forces g++ to use C++ 11 because older versions of g++ used an older standard of C++. C++ 11 is the minimum version required to compile Grub Customizer.
You can fix this by opening the file CMakeLists.txt and replacing "-std=c++11" by "-std=c++14".
Seems like Glibmm updated to C++ 14 now...
Grub Customizer forces g++ to use C++ 11 because older versions of g++ used an older standard of C++. C++ 11 is the minimum version required to compile Grub Customizer.
You can fix this by opening the file CMakeLists.txt and replacing "-std=c++11" by "-std=c++14".
... and I have to find a general solution.