"can be marked override" warnings for gzstreambuf functions: C++11 support?

[int-ua]

I'm trying to use this library as a git submodule and getting these errors:

##[error]libraries/gzstream/gzstream.h:69:21: error: ‘virtual int gzstreambuf::overflow(int)’ can be marked override [-Werror=suggest-override]
Makefile:915: recipe for target 'obj/cata_utility.o' failed
     virtual int     overflow( int c = EOF);
                     ^~~~~~~~
##[error]libraries/gzstream/gzstream.h:70:21: error: ‘virtual int gzstreambuf::underflow()’ can be marked override [-Werror=suggest-override]
     virtual int     underflow();
                     ^~~~~~~~~
##[error]libraries/gzstream/gzstream.h:71:21: error: ‘virtual int gzstreambuf::sync()’ can be marked override [-Werror=suggest-override]
     virtual int     sync();
                     ^~~~

https://github.com/CleverRaven/Cataclysm-DDA/runs/1111118107
Not sure if it's a gzstream problem but posting here just in case it is.