linux and _GNU_SOURCE define

[Roumen Petrov]

I find that adding -D_GNU_SOURCE is useful for linux.
This define enable more features from header files.

First) i found that define enable declaration/definition of following methods
- grantpt(...)
- unlockpt(...)
- ptsname(...)
for device "/dev/ptmx" ( support for ptys, preferred is method openpty (...) )

Second) _GNU_SOURCE enable use of utmpx and wtmpx ( support for login recording, 
preferred is USE_LOGIN and syslogin_write_entry )

Three) ? at this time i don't know.