gdhcp: Use gcc atomics instead glib's ones

gdhcp: Use gcc atomics instead glib's ones

g_atomic_int_exchange_and_add() has been removed from glib 2.30
and g_atomic_int_add() should be used. Though there are still
quite a few distros out which do not ship a glib version with
g_atomic_int_add().

Instead of maintaing a compatiblilty glib layer we just use
the built-in functions for atomic memory access.