Bug#914688: Default defines discrepancy

[Dimitri John Ledkov <xnox@ubuntu.com>]

xnox@diamond:~$ g++ -std=c++14 -dM -E -x c++ - < /dev/null | grep linux
#define __linux__ 1
#define __gnu_linux__ 1

xnox@diamond:~$ g++ -std=gnu++14 -dM -E -x c++ - < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1

The same is not observed on other architectures. And for some reason,
boost cares about "linux" or "__linux", and is not sensitive to
"__linux__" nor "__gnu_linux__".

-- 
Regards,

Dimitri.