Compile with -std=c11 along with _POSIX_C_SOURCE=200112L.

Compile with -std=c11 along with _POSIX_C_SOURCE=200112L.

With gcc 5.1 released and defaulting to std=gnu11, the code will be compiled
according to different standards depending on the compiler version so we
should better specify explicitly what standard we are targetting.
C11 is now quite mature, it is supported in the just release Debian stable
(gcc 4.9) and also in old-stable (gcc 4.7), so there should be no reason to
use anything more ancient.
We also should have no reason to need any non-standard GNU extension.
So using only C11 + POSIX sounds like the best option right now.