Clang >= 3.3 in c++1y mode cannot parse header

This note in the gets manpage looks relevant:

ISO C11 removes the specification of gets() from the C language, and since version 2.16, glibc header files don’t expose the function declaration if the _ISOC11_SOURCE feature test macro is defined.

Probably should be

#if !_ISOC11_SOURCE
using ::gets;
#endif