Ahmed S. Darwish authored Mar 14, 2016 and Tanu Kaskinen committed Apr 23, 2016

Per glibc feature_test_macros(7), setting compiler flags to
-std=c11 (or any c* variant like c99) enforces strict ANSI
mode.

Enforcing strict ANSI makes all declarations under _GNU_SOURCE
unavailable. This leads to build warnings in the form of:

    warning: implicit declaration of function ‘syscall’

Thus replace -std=c11 with -std=gnu11

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>