And then all that stuff at the end about language dialect "CXX14".
Target "supertux2_lib" requires the language dialect "CXX14"
Target "sexp" requires the language dialect "CXX14"
-- The C compiler identification is GNU 4.6.3
-- The CXX compiler identification is GNU 4.6.3
Ref:
C++14 Support in GCC
C++14 Language Features
https://gcc.gnu.org/projects/cxx-status.html#cxx14