Hello! We use CentOS 6 for compiling osgEarth too. 4.8.5 was insufficient for us.
https://gcc.gnu.org/projects/cxx-status.html#cxx11 suggests that 4.8 fully supports C++11 but our findings on 4.8.5 were different. We had consistent crashes on 4.8.5 and tracked it down to some header that defined the implementation, so it built, but the implementation was empty or aborted or something like that. I don't recall the details, but it was obviously wrong looking at the system header. I think it was something with std::thread or std::mutex but I don't recall for sure.
Our team upgraded the compiler using the devtoolkit package to gcc 8.x and everything went away. I think the 4.8 implementation of C++11 just didn't cut it. I believe 4.9 was OK, but I don't know for sure and we didn't do testing in that fine grained detail for what were just really old compilers.
It's unfortunate because the compiler reports that it supports C++11, but it doesn't seem to actually support the entire set, as per this bug. It sounds like you've found another instance where C++11 isn't fully supported by gcc 4.8.
It looks like CXX11.cmake only presumes correct support of C++11 for gcc>5. Ultimately for our team we decided that the 4.8 compiler was too buggy to keep using so we had to upgrade.