Compilation errors, when compiling pytorch on Ubuntu 17.10 or newer with GCC 5

This issue is related to Issue #5136 which describes problems with both GCC 6 and 5. Creating it separately as the problems are different and to be tracked separately.



The issue comes from a bug in `libstdc++-5-dev` package in Ubuntu 17.10 and newer. Macro `_GLIBCXX_USE_C99` is not defined in `/usr/include/x86_64-linux-gnu/c++/5/bits/c++config.h` which leads to errors related to missing members of **std** name space, such as `std::to_string`, `std::isnan`, `std::isinf` (and more). The bug is tracked under https://bugs.launchpad.net/ubuntu/+source/gcc-5/+bug/1725848 and won't be fixed as GCC 5 is considered outdated and not supported by Ubuntu any more.



It will be good if we shows a meaningful error when GCC 5 is used on Ubuntu 17.10 or newer.



**Note:** Cuda 8.0 support GCC 5 or older only. In case Cuda 8.0 is needed, current workaround is to use GCC 4.8 which has been confirmed to work.

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